The Angels announced that Pineiro likely will be out six to eight weeks. An MRI exam confirmed the diagnosis of team doctor Craig Milhouse: a strained left oblique muscle. Pineiro will be placed on the 15-day disabled list. He likely will be replaced in the Angels' rotation by rookie Trevor Bell, who was optioned to Triple-A Salt Lake on Monday.
Pineiro said he was midway through his pregame bullpen session when he felt a tugging on his left side.
"I was throwing a curveball. I just thought it was a cramp," Pineiro said. "I thought maybe I could stretch it out and see what happens. Then I tried to throw another and there was no chance, no way to throw it."
Angels manager Mike Scioscia got a call in his office about 12 minutes before the game was set to begin. He got word to the umpires and the game started three minutes after the scheduled start time of 12:35 p.m.
"It happens to teams all around baseball. It got us this afternoon," Scioscia said. "It's unfortunate. I think Joel was throwing the ball the best he has in his life right now."
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 warm-ups. He couldn't believe the extensive recovery time required.
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"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.'"
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.
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 on 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.
The Angels later announced they added a little bit of pitching depth when they signed ex-Pirate and Giants pitcher Ryan Vogelsong and assigned him to Triple-A Salt Lake. Vogelsong had been pitching for the Philadelphia Phillies' Triple-A team before being released July 16.
Before that, he spent three seasons pitching for the Orix Buffaloes in Japan.
Vogelsong was 10-22 with a 5.86 ERA in six seasons in the majors.
Mark Saxon covers the Angels for ESPNLosAngeles.com. Information from The Associated Press was used in this report.
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