But this time, it was different. Instead of joining the rotation -- his natural role, and one he's excelled at this season -- Masterson's return marked his arrival in the bullpen.
The move had been plotted as soon as Masterson was optioned to Triple-A Pawtucket on July 7. As it turns out, Masterson resurfaced about five days earlier than planned, thanks to David Aardsma being forced to the 15-day disabled list with a strained right groin.
The only thing the Red Sox weren't able to do with Masterson during his bullpen apprenticeship in the Minors was have him work on back-to-back days.
"I think he'll be fine," said Red Sox manager Terry Francona. "In fairness to him, I'm trying to not put a kid in a situation where he's got the bases loaded and two outs and [say], 'Here kid, first time, go get em'. We also have to remember, he's been in the bullpen for three outings. He's not going to go pitch five days in a row. That's not realistic. That's part of the reason we [would have] had him going back-to-back these next couple of days, just to get him used to it."
Now, Masterson will adapt to life as a Major League reliever on the fly. Despite a hectic night of travel on Saturday night -- Masterson was told on short notice to get on a flight from Providence, R.I., to Anaheim -- the righty was in his typically energetic mood before Sunday's game.
"It's a lot different," said Masterson. "I'm definitely going to have to pick the minds of the guys here in the bullpen and see how they go about it and all that stuff just to get an even better feel for it. It's a lot different when you come in and say, 'I might throw today, compared to knowing exactly when you're going to throw."'
At least early on, Francona will try to pick his spots with Masterson.
"I hope that the first time he pitches, we have a 10-run lead," Francona said. "I think we all think he can help us. The hard thing is, and you don't want to give guys too much, because it's hard enough to win in this league when you think you're a good team. All of a sudden you call up a lot of young guys and you give them too much responsibility, that's not good either. We just want to try to balance it as much as you can. Sometimes you get in a situation where you can't. I think he's got a mentality where he'll be able to handle what you give him."
With his nasty sinker, Masterson should be able to make a smooth transition during this stint as a reliever.
"I think it lends to any role," Masterson said. "It's definitely an opportunity to get lots of groundballs and that's what I want to do relieving or starting. I'd like to think I can be effective anywhere, so we'll see."
As for Aardsma, he suffered his injury during a July 8 outing against the Twins. He pitched once more against the Orioles on July 11 and the hope was that the All-Star break would be a cure-all.
After feeling good in a side session on Thursday, Aardsma was hindered by the groin injury during Friday's relief outing.
"His velocity has been down," Francona said. "He's been getting people out, but that's just not the way he needs to pitch."
Aardsma -- who is 3-2 with a 2.75 ERA in 38 games -- had no problem with the move.
"It was holding me back," Aardsma said. "We want to make sure we give it a lot of rest and I'm able to come back and be healthy."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.