The Cubs’ pitching mystery for Saturday is solved at last.

Brad Keller will open Game 2 against the Mets while No. 2 prospect Cade Horton will be ‘active’ to make his MLB debut out of relief, manager Craig Counsell told media in New York after Friday night’s 7-2 loss.

[MORE: Cade Horton, Cubs No. 2 prospect, ready to make long-awaited debut]

“We’re essentially running a bullpen game,” Counsell said.

Keller has a 3.78 ERA with 18 strikeouts in 16 2/3 innings of work out of the bullpen. He has been solid as of late, allowing zero earned runs in his last six appearances (6 1/3 innings).

Horton, 23, is the Cubs’ highly-anticipated top pitching prospect who falls at No. 46 overall on MLB Pipeline’s Top 100. He has been stellar in Triple-A this season, posting a league-best 1.24 ERA over 29 innings in his six starts.

For most of the past week, the Cubs held off on determining a starter for Saturday’s game after Shota Imanaga hit the 15-day IL with a left hamstring strain Monday. The injury was the second blow to the Cubs’ rotation, which lost Justin Steele to season-ending elbow surgery in April.

Horton finally appeared in the Cubs’ locker room at Citi Field on Friday afternoon after days of speculation that he would be moved to the big-league roster in Imanaga’s absence.

Horton told reporters that he didn’t look at Imanaga’s injury as an open door for him to make his debut.

“I really tried to focus this year on just being where my feet are,” Horton said. “I didn’t want to play GM, because when you play GM it just never works out that way.”

Horton has a career 2.79 ERA over 151 2/3 innings in the Cubs organization. His 2024 season was cut short due to a shoulder injury. Now healthy, the 2022 No. 7 overall pick said he didn’t set his heart on when he would be called up.

“I wanted it to be a cool moment and not expect it,” Horton said. “I was just focusing on controlling what I can.”

The Cubs will likely announce Horton’s official placement on the roster and the corresponding moves ahead of Saturday’s game. He’ll join a pitching staff eager to work with him, said Jameson Taillon.

“He’s an awesome dude — I’m really excited for him,” Taillon said of Horton postgame Friday. “He’s definitely earned this. He’s got a great pitch package, great stuff, great work ethic. I think everyone’s really excited to watch him tomorrow.”