The Milwaukee Bucks were rallying in the fourth quarter of Game 2 of their first-round series against the Chicago Bulls when a driving lane opened up for forward Khris Middleton.

Middleton made a beeline toward the rim before Bulls center Nikola Vucevic stepped in front of the basket. Middleton attempted to spin away, but his left knee buckled as he fell to the court. Middleton tried to walk it off, but once he exited the game and headed toward the locker room, the team ruled him out for the night.

Middleton was diagnosed the next day with a sprained medial collateral ligament in his left knee. He missed the first round and is expected to miss the entirety of the second round against the Boston Celtics, a series that is tied at 1-1 heading into Saturday's Game 3 (3:30 p.m. ET on ABC).

Middleton's absence poses one of the biggest challenges to the Bucks in their title defense.

The Bucks are 6-1 in the playoffs over the past two seasons when one of their Big Three -- Giannis Antetokounmpo, Middleton and Jrue Holiday -- is sidelined. But this round against the Celtics marks the first time they have entered a playoff series with an expectation to play short-handed.

Still, the Bucks' goal remains the same: Bring a second consecutive Larry O'Brien Trophy home to Milwaukee.

"We did it last year as a group when Giannis went down [in the conference finals against the Atlanta Hawks]," Middleton said before a team practice a few days following the injury. "Everybody stepped up and played a major part. I expect nothing less."

If the Bucks struggle to replace Middleton's production, it's more likely than not that they'll see their season come to an end early. But that's far from the only key to raising another banner. Here are three other crucial areas of focus, in addition to stepping up in Middleton's absence, that will drive the Bucks' road to success.

Giannis evolves again

Antetokounmpo provided an example of the kind of dominance that led him to win consecutive MVP awards in back-to-back road wins against the Philadelphia 76ers and Brooklyn Nets to close out the regular season.

To beat the Sixers in Philadelphia, Antetokounmpo collected 40 points, 14 rebounds, 6 assists and 3 blocks, including the game-saving rejection on a Joel Embiid layup attempt that could have sent the game to overtime. In Brooklyn two nights later, Antetokounmpo finished with 44 points, 14 rebounds and 6 assists. He hit a 3-pointer with 18 seconds left in regulation to force overtime, then drained two free throws with three seconds left in overtime to seal a one-point win against the Nets.

"If he can carry you like that," an Eastern Conference scout told ESPN, "I'm not sure anything else matters."

Antetokounmpo put up a career-high 29.9 points this season, which marked the fourth straight year he averaged at least 25 points, 10 rebounds and 5 assists, the most such seasons by any player in NBA history. He posted the highest field goal percentage (61%) in the league among those players, according to Second Spectrum.

"He just has that go mode where he's able to carry a large part of the offensive load for long stretches," Bucks guard Grayson Allen said. "Even if he's not scoring, it's running through him and he's the one creating opportunities for other guys. It's impressive he's able to keep that up for a long game and still get blocks and be active on the defensive end."

But after winning his first championship, the reigning Finals MVP returned even more dangerous this season, with improvements to the few weaknesses in his game. Antetokounmpo made 72% of his free throws during the regular season, his highest rate since 2018-19, while attempting a career-high 11.4 free throws per game. He shot 77% from the line in the final three games of the first round, but has slumped a bit against Boston, making 55% through two games.

Antetokounmpo also shot 36.1% on jumpers, the highest single-season rate of his career, according to Second Spectrum tracking. He continues to grow more comfortable in the midrange, taking 16.5% of his shots from there (up from 13% last season and 9.4% in 2019-20).

The superstar power forward also knows how to beat a double-team and create wide-open looks from 3 for his teammates. Antetokounmpo has assisted on 30 3-pointers during this postseason, the second most of any player (Memphis Grizzlies guard Ja Morant has 32), and 22 of those 3s have been uncontested (most of any player). In Game 1 against the Celtics, Antetokounmpo made 12 assists and posted the second postseason triple-double of his career.

"You keep thinking you're seeing a Giannis who is kind of scraping the ceiling, scraping the potential of what a guy can do. Then he comes back next year and he just does more," Bucks center Brook Lopez said.

"I've learned to kind of stop being like, 'Wow, he does everything,' because there's always something else he adds to everything. It's pretty infinite. It's cool and it all comes down to work ethic."

Reclaiming a defensive identity

The Bucks were an average defense during the regular season, finishing 14th in defensive rating, which usually foretells the end in the playoffs. No team ranked that low in defensive efficiency has won the title since the 2000-01 Los Angeles Lakers. That team, widely regarded as the ultimate "switch-flipping" team in NBA history, turned up the intensity in the postseason, leading all teams in defensive efficiency en route to a 15-1 run and a second consecutive title.

The Bucks have already lost once in each series and won't go unbeaten until the Finals. But like that 2001 Lakers team, they have demonstrated an ability to ramp up the defense when it counts.

"We gotta play defense to win," Antetokounmpo said following Milwaukee's victory over Atlanta in Game 5. "We don't know how many points we're going to score with Khris, one of our best scorers being out, but we gotta make it as hard as possible.

"We might score 90 points, but we might defend them and they can score 80."

In their first three seasons under coach Mike Budenholzer, the Bucks never finished lower than ninth in defensive rating. Last season, they were the best defense in the playoffs en route to the championship.

The Bucks certainly looked the part in the first round. They smothered the Bulls into 94.4 points per 100 possessions, by far the best defensive efficiency in the playoffs, while neutralizing Bulls star DeMar DeRozan, who shot 34% from the field in the first round. That trend continued in the second round when the Bucks held the Celtics to 89 points on 33.3% shooting in Game 1, before Boston caught fire in Game 2. The Bucks' 96.8 points allowed per 100 possessions this postseason is more than 6 points better than the next-best team (Miami Heat, 103.0) and would be the best defensive rating for a playoff team in more than a decade.

"We've been very good defensively for three years," Budenholzer said as the playoffs began. "There's a lot of good individual defenders, guys who just have that God-given talent, knack, focus. We just need to put it together. "

The return of Brook Lopez

A "Welcome Back" banner featuring Tigger from Winnie the Pooh hung from the Bucks' training room on March 14.

Lopez had started the first game of the season before missing the next five months because of surgery to repair a bulging disk in his lower back. The personal training staff that helped guide him through rehab decked out the room with his favorite Disney character.

"I'm so glad to be hooping with my guys again," Lopez said a few days after his return. "I missed that so much, the camaraderie on the bench, on the floor, and locker room and all that. And I feel great."

Even though he was limited to 13 games during the season, Lopez returned as the same rim protector who serves as the last line of Milwaukee's defense.

Since his return on March 14, Lopez held opponents to 39.5% shooting at the rim, the lowest field goal percentage among players who defended at least 40 shots at the rim, per Second Spectrum. The Bulls shot 37% from the field during the first round. He has blocked three shots in each of the first two games against Boston.

"It's just crazy to me that people overlook him," Holiday said. "Even last year, people overlooked Brook, where in Game [5] of the Atlanta series, he had like 30 [with Antetokounmpo sidelined]. He was dunking on everybody. But I do think people saw this year, without Brook, how much he means to our team, especially on the defensive end."

Making up for Middleton

Even though he watched the NBA Finals last summer from his couch, Wesley Matthews felt a connection to the Bucks' championship success.

Matthews was on the roster the previous season in 2019-20 -- on a Bucks squad that came up short in the bubble -- but he also grew up in Wisconsin and played in college at Marquette. So when an opportunity arose to rejoin the team 23 games into the season, Matthews was ready to play.

He signed with the Bucks on Dec. 3, came off the bench the next day and has made his way back into Milwaukee's starting lineup, once again tasked with the job of guarding the opposing team's best player.

"I wish my grandmother was here to see it," Matthews, whose grandmother, Madeline Gaines, died earlier in the season, said before the playoffs began. "That was one of the big things for me. Why I wanted to come back and why I fought so hard to come back in this league is for her to watch me again, to be in the playoffs and compete for a championship."

The Bucks spent the offseason trying to find a replacement for P.J. Tucker from last year's playoff roster, but they might have found their answer in a player who was sitting at home for the first two months of the season.

However, Matthews isn't the only player being counted on to step up in Middleton's absence. To close out the first round, Bobby Portis moved into the starting lineup and Allen caught fire shooting from 3 against Chicago (58.3%) to make up for some production offensively, while Matthews and Holiday hounded the Bulls defensively. Matthews was the most frequent primary defender on the team against DeRozan, according to research from ESPN Stats & Information, holding him to just 36% shooting. And the Bulls had no answer for Holiday's defense, shooting 29% with him as the primary defender.

"Khris clearly is a huge part of what we do, but we have a lot of talented guys on our team," Lopez said. "We're going to see a lot of different guys, I think, stepping up on different nights."