But Red Sox manager Terry Francona, who is the one who has ultimately put Masterson in those crucial situations, makes a valid point.
"He's earned it," Francona said. "Otherwise we wouldn't put him in those situations. He's earned it. For us to run away from him now, I think, would be an error on our part."
With Game 1 of the American League Championship Series on Friday night hanging in the balance -- the Rays had runners on first and second with one out in the eighth and down two runs -- Francona called on Masterson to face Rays cleanup man Evan Longoria.
The motive was obvious with the sinkerballer pitching. And Masterson executed perfectly, getting Longoria on a 6-4-3 double play that was perhaps the key sequence in a 2-0 win for the Red Sox.
"It's been a continual thing over the course of this year, whether I put myself in tough situations or come into tough situations, I think throughout the year it's been a good learning experience to help me get to where I am," said Masterson.
Where Masterson is right now is on the big stage performing a role he knew next to nothing about before the season started. In Spring Training, Masterson was billed as one of the top starting pitching prospects in the organization.
And he lived up to the hype, giving the Red Sox a solid nine starts (4-3, 3.67 ERA) when they had injuries to fill in the rotation during the first half. But as they continued to watch him pitch, Boston felt Masterson had the guts and the assortment of pitches to fill a hole in the bullpen. So he went back to Triple-A Pawtucket for a brief apprenticeship and resurfaced the second day after the All-Star break.
From then on, he became a mainstay in the bullpen, going 2-2 with a 2.36 ERA in 27 outings.
"He did everything we asked him this year," Francona said. "He started, he did great. [We] sent him back to Triple-A to go to the bullpen so we could be fair to him when he goes to the bullpen [for us]. He did exactly what he was supposed to do and came up and impacted our bullpen, which I think is how we all thought it would work out."
"His poise is tremendous. [Closer Jonathan Papelbon] was similar a couple of years ago. He came up in the middle of the year and impacted our bullpen in a huge way. I think we all agree that we feel pretty fortunate."
Francona was referring to what Papelbon did in 2005, when he made a similar conversion from starter to setup man.
Of course, Papelbon went on to become one of the most dominant closers in baseball after that, which creates the question: Will Masterson be a starter or reliever next season?
"Whoa, next year seems so far away," Francona said. "We'll have some fun when it's appropriate making those decisions. Regardless, he's going to be a good pitcher. Kind of like Pap, if you have enough good pitching, it will work."
In five outings in this postseason, Masterson has a 1.93 ERA.
The key to that success at such a young age?
"One, you can't dwell on what you've done," Masterson said. "You've got to come back and do your thing. Two, you're going to get thrown out there. You've got to be ready to go. That's where I want to be."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.