Vladimir Guerrero is out for at least a month

An offense that was already struggling to score runs received a major body blow Friday when Vladimir Guerrero was diagnosed with a torn pectoral muscle, an injury that sent the Angels slugger to the disabled list and will sideline him for at least a month and probably longer.

“Obviously, short of Vlad needing surgery, this is a big blow,” Manager Mike Scioscia said. “We’ll see how he progresses. Some of the windows we’ve been given [for how long he’ll be out] are optimistic; some are longer. But we’re going to be conservative. He’ll be reevaluated in a month, and we’ll see where he is.”

Guerrero, who suffered the injury while throwing from right field to third base in an April 2 exhibition in Dodger Stadium and was relegated to designated hitter for the first eight games, had an MRI test Thursday in Seattle. The initial diagnosis was a strain.

But Guerrero returned to Southern California to be examined Friday by Dr. Lewis Yocum, the Angels’ team physician, who reviewed the MRI results and deemed the injury more serious.

Though Guerrero, who was batting .250 with one home run and three runs batted in, said the injury didn’t affect his swing, Yocum recommended shutting down the team’s cleanup batter to give the muscle a chance to heal.

It now appears doubtful Guerrero, one of only two players -- Lou Gehrig is the other -- to hit .300 or better with at least 25 homers for 11 straight years, will be able to return to right field this season. The Angels are optimistic he will be able to return as a DH.

“At some point, he’ll be able to hit and play before he can throw,” Scioscia said. “Right now, Dr. Yocum feels the best course is to rest a bit, let it calm down.”

Guerrero underwent surgery on his right knee in October and didn’t begin exhibition play this spring until the third week of March, but he said the knee has been feeling good. He has not been on the disabled list since 2005, when he separated his left shoulder.

Batting third, at DH ...

On Friday night, it was Maicer Izturis. The utility infielder is hardly a prototypical No. 3 hitter -- he has 16 homers in 397 big league games -- but in Guerrero’s absence, Scioscia went for more of a situational look in hopes of setting the table for Bobby Abreu, Torii Hunter and Kendry Morales.

The move is not without precedent. Izturis batted third 14 times last season, and the Angels were 11-3 in those games. The switch-hitter had a .329 on-base percentage, and hit .319 with runners in scoring position and .444 (four for nine) with the bases loaded in 2008.

“We’re going to have to get creative, reconfigure some things and focus more on setting the table for Bobby, Torii and Kendry,” Scioscia said. “Izzy gives us more of a small-ball look. He’s shown the ability to get on base and drive in runs, and he has a real good approach with runners in scoring position.”

Scioscia said reserve outfielder Gary Matthews Jr. would get more playing time, and he did not rule out using the powerful Mike Napoli at DH on the nights he doesn’t catch. That move would necessitate the addition of a third catcher.

The Angels plan to make a roster move before today’s game to replace Guerrero. The likely candidates are infielders Brandon Wood, who is hitting .333 with three homers and four runs batted in at triple A, or Sean Rodriguez, who is batting .280 with three homers and 10 RBIs.

If the Angels add a third catcher, it would be Bobby Wilson or Ryan Budde.

Pitching in

The Angels’ biggest concern entering the season -- a rotation that was without the injured John Lackey, Ervin Saunders and Kelvim Escobar -- has been its biggest strength.

Entering Friday, Angels starting pitchers had combined for a major league-low 2.60 earned-run average.

“There hasn’t been a start when they haven’t given us a chance to win the game,” Scioscia said. “When we get our offense going, that’s going to translate to a lot of wins.”

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mike.digiovanna@latimes.com