Angels ace John Lackey is expected to be sidelined at least a month after he was diagnosed with a strained right triceps after an examination on Friday by Lewis Yocum, the team's orthopedist.
A magnetic resonance imaging confirmed the clinical diagnosis.
Lackey, who hasn't missed a start because of injury since breaking into the rotation in 2002, will be shut down without baseball activities for three to four weeks, at which time he'll be reevaluated. His earliest return, giving him several weeks or rehab, would most likely be in late April or early May.
The Angels, as a result, will open their American League West title defense without their twin aces.
Kelvim Escobar, recovering from shoulder inflammation, is making strides toward regaining enough strength to begin throwing and is expected to start the process sometime this week, manager Mike Scioscia said.
Lackey and Kelvim Escobar combined for 37 wins last year -- 19 by Lackey, a first-time American League All-Star and ERA champion in 2007.
"I'm going to do everything I can to get back as soon as I can," said Lackey, the club's Opening Day starter in 2007. "I'll be a little bit cautious. They'll be pulling me back a little slower than I want to go.
"They've been here before. I haven't."
General manager Tony Reagins said the club was "concerned" but also "somewhat optimistic in that [Lackey's absence] won't be too long."
Lackey, 29, said Yocum informed him that a triceps strain is similar to a hamstring strain. This ailment, behind the elbow, is in a different location than the forearm discomfort that had caused Lackey to get a late start in his spring workouts.
"You've got to let it heal on its own and be patient," Lackey said. "If I go out too early, I can end up setting myself back.
"It's no fun. It's definitely not where I wanted to be. I'll have to work hard, get the rest of my body stronger. It might be a little blessing if I'm a little stronger at the end of the season."