Here's the issue: Peacock has excelled as a reliever and a starter. As for McCullers, his 58 Major League appearances have all come as a starter.
Starting pitchers sometimes have trouble adjusting to a new warmup routine.
"We saw that it took [Francisco Liriano] a few weeks to get adjusted to that role," Astros general manager Jeff Luhnow said of the starter-turned-reliever he acquired from the Blue Jays on July 31.
"It's always a gamble. It's a little bit of an unknown. But you rely on the stuff being good enough. It's an adjustment that we're just going to have to roll the dice on."
He's committed to nothing except that McCullers will have a role.
"He has demonstrated his last couple of outings his stuff is ready to go," Luhnow said. "It's just a matter of how he fits in with the rest of the guys that we have. We're going to end up having starters in the bullpen, and we're going to want to use them."
What surely is tempting to Luhnow is that when McCullers was healthy for the first three months, he was among the best pitchers in the game.
McCullers combines one of the best curveballs on the planet with a 95-mph fastball. He was 7-2 with a 3.05 ERA at the All-Star break. But that was around the time his season began to come unraveled.
First, he attempted to pitch through a back injury when the Astros had four other starters on the disabled list. He missed August, made one September start and then was out another 18 days with arm fatigue.
He made two starts at the end of the season, and while the raw numbers weren't good (8.22 ERA), he felt good. He threw 83 pitches in the start at Fenway last weekend, and in that game, he was dominant for three innings before things came undone for a variety of reasons.
If McCullers is capable of throwing 83 pitches at a high level in a playoff game -- say five innings -- would switching Peacock back to the bullpen be a better option?
Or will the Astros wait until they get through Games 1-2 and see if they've needed Peacock in relief? All in all, a good problem to have.
"Lance is a very talented pitcher, and he has weapons," Astros manager A.J. Hinch said. "And the league knows it, our team knows it and Lance knows it. So I think he could very much pitch in a variety of roles, including starting.
"Where we deem that the most important and what outs we think are going to be the most important is going to be discussed over the next couple days -- or next day, communicated to him and then I'll put him in there."
"What was asked of me coming off my injury mid-September was to show them I have my [velocity] back and I was able to throw without any pain or holding anything back," he said. "I think I did that."
He also makes it clear he's OK with a relief role if that's how the Astros feel he'd best help them.
"When you're called upon, regardless of what role it's in, you have to be ready," he said. "However they decide to use me this round -- it could be different going forward -- if my name's called on regardless of what role it's in, the team is going to be counting on me to do a job, and I'm going to have to step up and do it."
Richard Justice has been a reporter for MLB.com since 2011. Read his columns and follow him on Twitter at @RichardJustice. This story was not subject to the approval of Major League Baseball or its clubs.