When Justin Steele was injured on opening day, the Chicago Cubs’ primary issue was how their rotation would fare without its ace left-handed pitcher.

That anxiety vanished with time. Steele returned to one of baseball’s top rotations on Monday night, starting Chicago’s 6-3 loss to San Diego in the first of a three-game series. He allowed three hits, two strikeouts, and one walk in 4 2/3 shutout innings at Wrigley Field. Steele made his first major league appearance since March 28, when he hurt his left hamstring while attempting to make a sacrifice bunt at Texas.

Steele said it felt great to pitch at Wrigley again in front of the crowd. “It’s very fantastic to be back with the team, you know, in the locker room, hanging out with the boys, and all that. In terms of how I felt and so on, I felt great.”

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 threw 68 pitches against San Diego, 43 for strikes.

We got to build up some endurance still,” manager Craig Counsell said. “But for where we were at, I thought he delivered a great performance. Exactly what we needed, and a great first step.

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 have a 21-15 record, more than holding their own while Steele was out.

Chicago’s strong start has been largely due to its rotation. Shota Imanaga, who will pitch on Tuesday night, is 5-0 with a 0.78 ERA in his maiden 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 outstanding, with rookie right-handers Hayden Wesneski and Ben Brown also contributing vital innings.

“I think our starting pitching has been excellent,” 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.”

Chicago’s starting pitchers have tossed 23 scoreless innings over the last four games. The rotation has a 3.04 ERA on the year, ranking third in the major leagues.

“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.”