Daisuke Matsuzaka, coming off two outstanding starts, left in the fifth inning with elbow tightness after his velocity fell off from the start of the game. The injury doesn't appear serious, but he will be checked out more on Saturday.
"I think we think he's going to be OK for his next start," manager Terry Francona said.
Bobby Jenks entered in the seventh to protect a 4-3 lead and couldn't do it. The struggling reliever gave up two runs and three hits, leaving Boston with its third loss in four games.
Jenks, signed as a free agent by the Red Sox during the offseason, began his Boston career with four hitless outings, but since then has appeared six times, working 4 1-3 innings and allowing 12 hits and eight earned runs.
"I think it's location," Francona said. "His stuff is good. He's got all the pitches."
Jenks (1-2) can't figure out why he feels so good and is having such bad results.
"I don't even know what to say right now," he said. "I'm going out there, feel terrific, all my stuff is there, just the results aren't. I don't know what to make of it."
Asked if this has happened to him in the past, Jenks said, "Not this many times in a row. It seems like every time I'm going out there right now everything's just seeming to find a hole or a flare in somewhere. I mean, they are good hitters, but the way I'm feeling right now it shouldn't be happening.
"I don't know where I'm at right now. Obviously, something's not clicking," he added. "If something's off I've got to fix it and fix it now."
Mike Cameron hit two solo homers for the Red Sox, his first home runs since July 18. He ended a stretch of 54 homerless at-bats, and his second homer gave the Red Sox a 4-2 lead through five innings.
Boston's bats then fell silent. The Red Sox didn't get a hit for the final 4 2-3 innings against three Seattle pitchers, led by starter Jason Vargas (1-2), who won for the first time since August. The three pitchers retired the last 13 Boston hitters (getting 14 outs) as the Mariners earned their fourth straight win.
"Their guy (Vargas) has a real good changeup and then he throws that 87, 88 mph fastball enough and locates," Francona said. "He knows how to pitch. He got a lot of our guys out in front with the changeup and then went to the fastball a little bit later, second and third time through."
NOTES: The manual scoreboard in left field spelled out "Go Bruins" and "Go Celtics" before the game. ... Struggling Red Sox LF Carl Crawford was given the night off. Crawford is batting just .160 this season and Francona thought it was a good opportunity to get some right-handed hitters a chance against Vargas because of a string of lefties the Sox are due to face. ... Kevin Youkilis, who jammed his left hip Thursday night in Baltimore, was able to play. ... Friday marked the 25th anniversary of the night Roger Clemens struck out 20 Mariners in a game at Fenway Park.