The team will wait until after Masterson completes a bullpen session on Sunday to decide on his next step.
"There's a definite progressive step being taken here," Red Sox manager John Farrell said. "We'll probably wait until he throws his bullpen [session] tomorrow to determine if there's another one needed or we insert him back with us. He got to 80 pitches last night, which is the minimum somebody's got to throw on a rehab [assignment] to come back in a rotation spot."
Farrell said the club has not determined whether Masterson could eventually be moved to the bullpen.
Masterson allowed five hits and hit a batter during the first two innings before settling down for the remainder of Friday's outing. The sinkerballer had allowed three runs over 1 2/3 innings in his first rehab start with Triple-A Pawtucket.
"In talking with [pitching coach] Kevin Walker at Portland, there was an increase in consistency to the action to his sinker," Farrell said, "but that really started to settle in in the third inning. First couple of innings, it was kind of getting his feet on the ground and get a feel for the game. The next step, we wouldn't want that to show up in a Major League game."
Masterson is 2-2 with a 6.37 ERA in seven starts.
Alec Shirkey is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.