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Angels right-hander Joel Pineiro will miss six to eight weeks after straining a muscle in his left side while warming up for a start against the Boston Red Sox on Wednesday, dealing another blow to Los Angeles' chase of another playoff berth.

Pineiro hurt an oblique muscle while throwing in the bullpen. After an MRI exam, the Angels immediately announced he'll be out for well over a month while putting him on the 15-day disabled list.

"I just threw a curveball, and a lot of pain came through the oblique," Pineiro said. "It's weird. I have no idea how to explain it. It was just one of those weird things that happens. I'm really frustrated and angry that this happened right now, at this moment."

The injury further hampers the Angels' attempts to get back in contention for their sixth AL West title in the past seven seasons.

A 7-3 loss to Boston on Wednesday dropped the Angels into third place in the division, nine games behind leader Texas. The Angels have lost seven of eight, and the Red Sox ended Los Angeles' streak of 79 straight home series without being swept dating back to June 2007.

Pineiro has been a dependable starter in his first season with Los Angeles, going 10-7 with a 4.18 ERA. He had won seven straight decisions before losing his most recent start in New York last week.

Pineiro even had two extra days of rest since his last start, but hurt himself midway through warmups. He couldn't believe the extensive recovery time required.

"My heart just dropped to the ground," Pineiro said. "'There's no way. There's got to be a mistake,' I told them. They said: 'You've got a pretty good strain in there, but everybody recovers differently, so we'll see what happens with you.'"

Veteran Angels reliever Scot Shields struggled while making just the 15th start of his major league career against Boston, giving up two homers and two walks in his last-minute assignment before leaving in the second inning with the bases loaded. The longtime middle reliever hadn't started a game since Sept. 28, 2003, against Texas.

After relievers Francisco Rodriguez and Rich Thompson combined on 5 1-3 solid innings, Fernando Rodney gave up Marco Scutaro's decisive grand slam in the eighth inning.

"I felt like I should take that loss, no matter whether I was out there or not," Pineiro said. "Those guys in the bullpen tried to pitch their hearts out. It was unexpected — 10 minutes before the ballgame — but those guys did what they had to do."

The Angels' once-solid starting rotation suddenly looks shaky. Left-hander Scott Kazmir is out with shoulder fatigue, and Los Angeles traded steady Joe Saunders to Arizona last Sunday in a deal to land three-time All-Star Dan Haren, who was hit on the right forearm by a line drive on Monday.

Ace Jered Weaver and Ervin Santana both are healthy, and Haren said he'll make his scheduled start Saturday against the Rangers.