In a corresponding roster move, the Sox recalled right-hander Devern Hansack from Triple-A Pawtucket.
It has been a tough year from the start for Timlin, who suffered a strained left oblique in late February and missed the bulk of Spring Training.
Timlin began the year on the DL before being activated on April 10. The 41-year-old veteran, who also struggled with arm problems last year, had been unable to get into a rhythm since being activated.
"It's been there for a while," said Timlin. "It's something you accept when you throw a baseball. It's not a natural motion. [You] deal with small aches and pains here or there. We just decided it was a good opportunity to let this cool down."
Timlin posted a 6.23 ERA over nine outings, and gave up at least a run in four of his appearances, including Wednesday's save over the A's.
He will accompany the Red Sox on their road trip to Minneapolis and Toronto.
"[I'm going to] stay with the team, put my pom-poms on and go get them," said Timlin.
According to Timlin, he actually could have pitched on Thursday night if the team hadn't placed him on the DL. But the brass took that decision out of his hands.
"I could have gone again today," said Timlin. "I told [pitching coach] John Farrell that and Terry [Francona]. It's their way of keeping me in check. If my arm hurts and the team needs me, I'll throw."
But the earliest he'll be able to throw again -- at least in a game situation -- is May 18.
Hansack had been starting at Triple-A Pawtucket, where he went 1-3 with a 3.77 ERA. Over 28 2/3 innings, Hansack had 34 strikeouts.
The Red Sox think highly of the 29-year-old from Nicaragua. Hansack was added to the roster late last September, going 1-1 with a 2.70 ERA in two starts. In the regular-season finale, he pitched five no-hit innings in a game that ended at that point because of rain. Hansack has the versatility to start or pitch in relief. More than likely, he will pitch out of the bullpen.
Lester checks in: It was just a cramp that took Jon Lester out of his Triple-A start after just three innings on Wednesday night. Nonetheless, it figures to put at least a small crimp in Lester's pitching schedule.
"It was a cramp," said Lester. "Nothing serious. Just see what the doctor says, maybe take a day or two off, maybe push back the start a day or two and go from there."
Lester, who had his 2006 season cut short after being diagnosed with a rare form of non-Hodgkin's lymphoma, experienced the cramp in his left forearm and said it has happened to him several times in the past, but usually the day after he pitches.
He came to Fenway Park on Thursday and had a precautionary check-up with Red Sox medical director Thomas Gill.
"[It was] one of those things that happened and there's nothing I can do about it," said Lester. "Hopefully it's not too long and everything just continues to get better. I've always had it throughout my career. It's just one of those things. It's my fault for not hydrating and being prepared."
Lester's 30-day rehab assignment expires on Friday. Though the Red Sox have yet to make anything official, they will more than likely option Lester to Pawtucket so he can continue to build up his endurance. He would have gone 95-100 pitches on Wednesday if not for the cramp.
Pena in, Drew out: After being scratched from Tuesday's game with viral symptoms, J.D. Drew returned to the starting lineup on Wednesday night and went 1-for-5. But with a lefty pitching for the Mariners on Thursday, Drew was back on the bench, replaced by Wily Mo Pena.
It turned out to be a solid move, with Pena going 4-for-4 to raise his average 101 points.
"I was talking to [Drew] after the game, and I don't think he feels real good yet," said Red Sox manager Terry Francona. "I wanted to find a way for Wily Mo to play tonight against [Seattle's Horacio Ramirez], and I think it kind of covers both birds with one stone. Give [Drew] another day to feel good about himself and we'll get Wily Mo's bat in there. I was probably rushing to get him back yesterday, but I was so glad he said he felt good. I probably should have given him the day yesterday."
Well-deserved day off: There was one guarantee heading into Thursday night's game. Lefty reliever Hideki Okajima would not be pitching. Okajima leads the staff with 14 appearances and had pitched in four of the last five games.
"I actually didn't think he was as sharp [Wednesday] night as he's been, and that's my fault, not his fault," Francona said. "I don't want to let that happen. The guy is getting everybody out, it's just so easy to want to run him out there a lot."
Almost as if to take away the temptation, Francona declared Okajima unavailable practically the moment Wednesday's game ended.
"One of the first things I told him when he came through the line last night was, 'You're not pitching tomorrow.' He nodded," said Francona. "I'm not sure if he knew what I was saying. He will when he doesn't get the ball."
It was J.C. Romero, the team's other lefty reliever, who wound up getting the save on Thursday. Closer Jonathan Papelbon was still unavailable after throwing 35 pitches on Tuesday.
On deck: Knuckleballer Tim Wakefield will open up a six-game road trip for the Sox when he takes the ball Friday night at Minnesota. He'll be opposed by Carlos Silva, who is 2-1 with a 3.10 ERA. First pitch is slated for 8:10 ET.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.