It wasn't his last stand for a big league role, not with a Tigers bullpen long on opportunities and short on established arms. But with no guarantee of a better situation, this was his chance to impress some minds.
Three scoreless innings later, as the Tigers headed home with a victory and their first three-game series win on the road since early July, Krol made an impression.
"I'm hoping it actually might be a little bit of a turning point for him," manager Brad Ausmus said.
It was advancement out of necessity for Krol, and it started with a 3-0 pitch. It couldn't simply be a get-me-over pitch, not with Mauer. It had to be well placed.
"He was pretty patient with me the whole at-bat," Krol said. "I mean, I threw some good cutters in there and he laid off of them. But that 3-0 pitch, I kind of just wanted to get it over the plate."
It was a 96-mph fastball up and on the outer corner, after he had missed low and in on back-to-back sliders.
"Once I found my balance point, it seemed like I had it all come together for the rest of that at-bat," Krol said.
Mauer took it, just as he took the 3-1 slider at the knees to run the count full. With the count full, Krol challenged him, delivering a 97-mph fastball over the plate. Mauer was late on it.
"It was definitely a confidence booster," Krol said. "I think the whole outing for me is different, completely different [after that]. It changes every aspect of how I pitch the rest of the time I'm out there."
Krol threw seven consecutive strikes from the 3-0 pitch to inning's end. He overpowered Eduardo Nunez with three straight fastballs, getting a groundball out, then stranded the winning run at third with a 97-mph fastball that Shane Robinson popped out to center.
"That's a high-leverage situation right there," catcher James McCann said. "Him coming out and having the success that he had, you hope he can build off of that."
He retired three right-handed hitters in order in the 11th, including Kennys Vargas on a called third strike. He did it so efficiently that Ausmus asked him on his way in if he felt good enough to pitch the 12th, lead or tie.
"[Bench coach] Gene [Lamont] joked, 'You got six in you,'" Krol said.
Once the Tigers scored three, he didn't need to worry past that. After a one-out walk to Aaron Hicks, all he had to do was worry about Brian Dozier. He couldn't hurt Krol, but Mauer as the potential tying run could.
Krol wasn't tired at that point, not even with his pitch count approaching 40, but he was losing grip on the ball. He went from 0-2 to a full count before getting a game-ending double-play grounder.
Krol delivered nine outs in as many pitches (39) as he needed to record four outs on June 17 at Cincinnati. He also delivered his case for a role next season.
"It was a lot of fun," Krol said. "It was also very exhausting."