CHICAGO -- When Justin Steele was injured on opening day, the big question for the Chicago Cubs centered on the state of their rotation without their ace left-hander.

That concern faded away over time.

Steele returned to one of baseball's best rotations on Monday night, starting the opener of a three-game series against the San Diego Padres. It was Steele's first major league appearance since he strained his left hamstring while making a play on a sacrifice bunt at Texas on March 28.

The 28-year-old Steele was a breakout performer last year, going 16-5 with a 3.06 ERA in 30 starts. He made the NL All-Star team for the first time and finished fifth in balloting for the NL Cy Young Award.

Steele made a rehab start with Triple-A Iowa on Wednesday, allowing three runs and six hits in 3 1/3 innings. He was working on a pitch count against San Diego.

Keegan Thompson was optioned to Iowa to make room on the roster. Thompson is 1-0 with a 3.38 ERA in seven relief appearances for Chicago this season.

The Cubs had a 21-14 record going into the series against the Padres, more than holding their own while Steele was out.

The rotation has played a key role in Chicago's fast start. Shota Imanaga, who pitches on Tuesday night, is 5-0 with a 0.78 ERA in his first big league season after signing with the Cubs in January. Jameson Taillon (3-0, 1.13 ERA) and Javier Assad (3-0, 1.66 ERA) have been terrific, and young right-handers Hayden Wesneski and Ben Brown also have provided some valuable innings.

"I think our starting pitching has been excellent," manager Craig Counsell said. "Obviously some of the guys that were in the rotation all year, Shota and Javy, have been brilliant, and Jameson Taillon's been excellent, and then Hayden and Ben have come up and been really important."

Going into the matchup with San Diego, the starting staff had a 2.61 ERA in the team's past 21 games going back to April 14 -- the second-best mark in the NL over that stretch, trailing only Philadelphia (2.53).

"I think all those guys have stepped up and really produced for us," president of baseball operations Jed Hoyer said. "And that's what you need to get through injuries."