Masterson, who is 4-3 with a 3.67 ERA for Boston this season, will return to the PawSox in a slightly different role -- he'll be coming out of the bullpen. Outfielder Jeff Bailey joined the Red Sox to fill Masterson's spot on the 25-man roster.
"We wanted to, at some point, get Clay back here and get into the rotation," manager Terry Francona said. "We also think getting Justin acclimated to the bullpen really gives us a chance to really help our club."
Francona said Masterson will pitch for Pawtucket on Wednesday, Friday and Sunday, with hopes that he can make an impact for Boston later in the season in a relief role. And though he pitched rather effectively in his two stints with the Sox this season, Masterson said he knew the move would eventually happen.
"I think if you're realistic, everyone knew the time was going to come when guys would be coming off the [disabled list], and they'd be going back to their roles," Masterson said. "We have such a good pitching staff. I think it's nice that they think I can help them sometime down the road through the bullpen."
Francona made several allusions to how effective he thinks Masterson can be out of the bullpen, especially against right-handed hitters.
"He's very good at giving confidence, so with those words, it excites me just to see what will come out of it," Masterson said.
Buchholz will likely be recalled on Friday to take Masterson's spot in the rotation.
Meanwhile, Buchholz is 4-2 for the PawSox with a 2.47 ERA. He allowed just one run in his four stars between June 14-30, but he was touched up for five runs in five innings against Triple-A Lehigh Valley on Saturday.
Buchholz went on the DL in May with a broken finger nail on his throwing hand, then found himself in Pawtucket for an extended period of time, with Masterson and Bartolo Colon having strong stints as part of the rotation.
The time in Pawtucket allowed Buchholz to work on his fastball command and a small adjustment to his arm angle. Francona said he and his coaching staff felt not rushing Buchholz back was the best move in his development.
"When he was going to come back, we decided at that point to look at the big picture," Francona said. "It's not easy to do sometimes, because getting outs is at such a premium. The kid throws a no-hitter and it looks very easy, but there is still a lot of developing left to do."
Mark Remme is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.