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."
Scioscia, thinking long-term, made it clear the club would be pushing no panic buttons and making rash decisions. The Angels, leading the AL West every day of the 2007 season after April 24, won the division for the third time in four years, comfortably besting Seattle by six games .
The Mariners have beefed up their rotation with the additions of Erik Bedard and Carlos Silva, but Scioscia is confident he has the depth to withstand any division challenges.
"Right now, we're being tested," Scioscia said. "I don't think anyone could have said definitively where we'd be without John Lackey and Kelvim Escobar ... We're going to be fine.
"Even without [Lackey and Escobar], we have a rotation that can go out and win on a nightly basis and a bullpen that can hold leads, and an improved offense. We can win ballgames, and we're going to go out and do that."
The club currently plans to replace Lackey from inside the organization.
First call goes to middle reliever Dustin Moseley, who made eight starts last season, including a pair of impressive ones to open the season with Jered Weaver and Bartolo Colon sidelined. Moseley had surgery after last season on his ulnar nerve and has pitched three innings this spring, giving up two runs.
Also in the mix, Reagins said, are right-handers Nick Adenhart, Nick Green and Kasey Olenberger.
The most experienced of the three is Olenberger, a 29-year-old right-hander who has gone a combined 17-12 the past two seasons at Triple-A Salt Lake.
Green, 23, is 30-22 with a 3.92 ERA in 99 Minor League games, 74 as a starter. Adenhart, 21, is 28-15 in 64 Minor League starts with a 3.11 ERA. Both have pitched no higher than Double-A Arkansas and were expected to start the season in Salt Lake's rotation.
"Right now, we think we have the candidates internally," Regains said. "We think we've built some depth in the organization and have some young kids who are exciting.
"We think the timetable for Escobar and Lackey ... they're going to miss part of the season, but we don't think it will be long-term."
As for looking into free agency for a starter, Scioscia indicated that would be a backup option because "you have to understand, if there's one commodity every team in baseball is looking for, it's pitching."
Weaver, Jon Garland, Joe Saunders and Ervin Santana form four-fifths of the rotation. Garland's acquisition from the White Sox in November at the cost of shortstop Orlando Cabrera looks even better now.
Adenhart assumed Lackey's starting assignment on Saturday at Tempe Diablo Stadium, facing the Diamondbacks' Randy Johnson.
Adenhart, a 6-foot-3 athlete with a dominant fastball and big curveball, has been impressive in Cactus League play, yielding three runs in 8 2/3 innings.
In his most recent outing, Adenhart went four scoreless innings behind Lackey on Monday, gaining the win against the A's by allowing one hit and striking out three with no walks.
Lackey pitched 1 2/3 innings in his spring debut, reporting no pain in the elbow afterward. But there was soreness the following day, and the pain became more intense following a side workout on Wednesday.
"I felt good during that game," Lackey said. "It took me a little longer to come back. I was pretty sore, and after my last bullpen session, it didn't go away.
"It's the first time I've missed a start since I got drafted [in 1999]. It's definitely disappointing, but I guess it could have been worse."
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.