BOSTON -- Celtics star Kemba Walker has left knee soreness and will miss Saturday night's nationally televised showdown with the Philadelphia 76ers, coach Brad Stevens said.

"Kemba's out," Stevens said following the team's morning shootaround at their practice facility prior to Saturday's game (8:30 p.m. ET, ABC). "Most likely, at minimum, the next two games, maybe, and then we'll reevaluate it after that."

Stevens said Walker's knee began to bother him Thursday, when he played 31 minutes in a 119-104 victory over the Golden State Warriors. He had played at least 35 minutes in each of the prior three games -- the second time this season Walker had played at least that many minutes in three consecutive games.

"He was very sore yesterday," Stevens said. "He tried to give it a go today, but he's not playing."

Walker sat out of Boston's win over the Phoenix Suns two weeks ago with soreness in the same knee, and he also had an MRI, which came back clean.

Stevens said Saturday morning that while the Celtics don't expect it to be a long-term issue, it is something the team could have to manage moving forward.

"I don't know what they've officially labeled it as," Stevens said, when asked if Walker had been diagnosed with tendinitis, "but it's been ongoing all year. And he missed I think a game earlier this month for it. But it flared up.

"You could tell he didn't have his normal burst the other day. I don't think it's anything that we think is a long, long-term thing, by any means, but we certainly may have to manage it."

The Celtics have made it a point to manage Walker's minutes all season -- he's averaging 32 minutes per game, his lowest average since his rookie year, when he played 27 minutes a night.

Walker, who was named to his fourth straight All-Star Game last week, will miss his sixth game of the season Saturday night -- the same number he missed in total over his last four seasons with the Charlotte Hornets, including playing all 82 games last season.

Boston is likely to start Marcus Smart against the Sixers, whom they've already lost to three times this season. If Philadelphia wins Saturday night, it will be the first time the Sixers have swept their rivals since the 2000-01 season -- which also happens to be the last time Philadelphia reached the NBA Finals.

While Walker is out for the Celtics, the Sixers will get back Al Horford, who missed Thursday's loss in Atlanta with knee soreness of his own. Horford will make his official return to Boston, where he played the past three years, after sitting out with knee soreness the first time the two teams met here in December.

"A lot happened in three years, on the court and off the court," Horford said. "All I have is positive memories of my time here."

In addition to Walker, the Celtics will also continue to be without both Enes Kanter and Robert Williams, leaving them short-handed at center as they go against Joel Embiid. As a result, the Celtics also called up Tacko Fall from the G-League's Maine Red Claws on Saturday, and he is likely to be active for the game.