It turned out to be his slider, and not by just a little.
"This was probably the first one I didn't really feel the greatest," Masterson said. "It happens. It's going to happen again - probably multiple times. You just try and pitch through it."
Masterson (7-3) was trying to become the second eight-game winner in the majors. Instead, he was pulled after throwing a season-low 93 pitches, allowing five runs on six hits. He also walked one and hit a batter, missing so badly with a slider that it struck Daniel Nava's back foot in the sixth inning.
"I didn't do as well as I was hoping to. The guys probably wish they would have put some more runs on the board," Masterson said. "It was raining. It was ugly."
Masterson didn't blame the rain, which delayed the start of the game by 44 minutes. Boston starter John Lackey (3-4) was having no control issues.
"He did a tremendous job tonight," Masterson said.
Lackey, who missed all last season following Tommy John surgery, allowed two hits in seven innings in his second straight strong performance. He struck out eight, walked three and gave up an unearned run while pitching through a steady rain.
The Indians fell out of first place in the AL Central and a half-game behind the Detroit Tigers, who beat the Minnesota Twins 6-0.
"Regardless of what happens, tonight's game can have no bearing on tomorrow and that's what we'll do," said manager Terry Francona, who was greeted warmly once again by Boston fans.
The Red Sox remained one game behind the AL East-leading New York Yankees, who beat the Tampa Bay Rays 9-4.
Carp broke an 0-for-21 slump in the second inning with his third homer of the year. That also ended a 20-inning scoreless streak for Masterson. The homer followed a walk to David Ortiz and a single by Mike Napoli.
Francona received an ovation when he walked back to the dugout after making two pitching changes in the seventh, when Boston scored four runs. Francona returned to Fenway Park as an opposing manager on Thursday night for the first time since he was let go by the Red Sox after the 2011 season. The Indians won that game 12-3.
After Carp's homer, the Indians made it 3-1 in the third with the help of a wild throw by catcher Jarrod Saltalamacchia. Singles by Reynolds and Michael Brantley put runners at first and second before Lackey retired the next two batters. On a double-steal attempt, Saltalamacchia threw the ball into left field, allowing Reynolds to score and Brantley to reach third.
The Red Sox made it 4-1 in the sixth when they loaded the bases with one out. Daniel Nava was hit by a pitch and Dustin Pedroia and Ortiz singled. Napoli then grounded to shortstop Cabrera, who threw to second baseman Kipnis for a forceout. Kipnis' throw to first pulled Nick Swisher off the bag as Napoli reached safely and Nava scored. Pedroia also tried to score, but Swisher threw him out.
Boston took an 8-1 lead in the seventh on two-run singles by Jacoby Ellsbury and Pedroia.
- Giambi is hitless in his last 26 at bats after going 0-for-4.
- Cleveland activated Swisher off the paternity list and optioned IF Cord Phelps to Triple-A Columbus.
- Cleveland's Scott Kazmir (2-2) faces Jon Lester (6-1) on Saturday in the third game of the four-game series.