PITTSBURGH -- Ready or not, here comes Mitch Keller.

Keller, the Pirates' No. 1 prospect per MLB Pipeline, will make his Major League debut in the second game of Pittsburgh's doubleheader against the Reds on Monday at Great American Ball Park in Cincinnati, the club announced Sunday morning.

“Perfect world, we’d have given him some more time. We’re not in a perfect world,” Pirates general manager Neal Huntington said. “We feel like he’s ready to come up here and compete, and he’ll have the opportunity to go out there and continue to grow.”

Pittsburgh was initially hesitant to call up Keller from Triple-A, as he only recently added a hard slider to an arsenal that also includes a fastball, curveball and changeup. The Pirates wanted Keller to get comfortable with his new offering and learn how it complements his other pitches away from the spotlight of the Majors. On May 5, Huntington said it would have been like calling up a hitter while he’s overhauling the mechanics of his swing.

Recent circumstances changed the situation, especially with a doubleheader on deck. So did Keller’s performance.

“To bring him up on May 5, as he was adding the slider, just wouldn’t have been fair to him. He now has confidence,” Huntington said. “He now can shape it more consistently. He now has a better feel of when and how to use it. Now we have to go see how he evolves in the Major Leagues.”

The Pirates enter Monday without Opening Day starter Jameson Taillon, right-hander Trevor Williams and a handful of relievers. Left-hander Steven Brault and righty Nick Kingham struggled in their transition from the bullpen to the rotation, though they’ll each get a start in Cincinnati, along with Keller.

While that strategy has taxed the Bucs’ beleaguered bullpen, Keller has seemingly turned a corner in Triple-A. On May 14, he held Scranton/Wilkes-Barre to two runs on three hits and struck out eight over six innings. On Tuesday, he struck out six and allowed only one run over a season-high seven innings against Louisville.

“He’s continued to learn things as he’s gone on,” manager Clint Hurdle said. “Sometimes the game’s been easier. Sometimes the game’s been hard. That’s all part of your developmental curve. Nothing’s linear in this game.”

The Pirates drafted Keller out of Xavier High School in Cedar Rapids, Iowa, in the second round of the 2014 Draft. He worked his way through their system and began last season with Double-A Altoona, where he posted a 2.72 ERA in 14 starts. He earned a promotion to Triple-A in late June, started the SiriusXM All-Star Futures Game and then thrived in his final eight starts for Indianapolis, posting a 3.09 ERA.

Keller reported to Major League Spring Training this year and struggled, giving up 10 runs on 10 hits in four innings over three appearances. Club officials repeatedly emphasized that fans did not see the “real Mitch Keller” in Spring Training, but they hope that he learned from the experience.

“It was probably a great opportunity for him to understand that he’s going to have to make pitches,” Huntington said. “The breaking ball, the hanger that he gets away with in the Minor Leagues, he’s probably not going to get away with as much up here.”

The Pirates’ pitching depth has already been tested in a way they didn’t expect. The bullpen, which might have been well-equipped for the opener strategy at full strength, lost a handful of key members due to injuries or ineffectiveness.

So here comes Keller for now, and what comes next is unclear.

Keller’s time with the Pirates might be brief. Huntington wouldn’t say if Keller will be added to the 25-man active roster or recalled as the designated 26th man for Monday’s doubleheader. The Pirates are sincere in their desire for Keller to get more seasoning in the Minors, but their rotation won’t return to full strength anytime soon.

“We’ll see how a series of things go,” Huntington said. “I don’t want to say he’s on a start-by-start basis, because there are some things beyond his control in terms of how other guys do and how the rest of this roster plays out. But he could be one [start] and back to the Minor Leagues to continue to grow and develop, or he could get an extended opportunity. He also may never go back.”