How long he'll be out, if he'll miss any time at all, is expected to be determined by Yocum. An MRI has indicated no structural damage.
Torii Hunter assumed Guerrero's cleanup spot with Juan Rivera taking over as designated hitter, where the slugger has been slotted through the team's first eight games of the season. He was trying to get past the pain that first surfaced in an exhibition game against the Dodgers on April 2.
"We've talked about this thing in his pectoral," Angels manager Mike Scioscia said. "He hasn't been able to throw. We had an MRI, and it's diagnosed right now as a strain. He's going to be re-evaluated by Dr. Yocum tomorrow and take it one day at a time."
Guerrero, hitting .250 with one homer and three RBIs in 32 at-bats, said the muscle strain hasn't bothered him swinging the bat.
"After the game [Wednesday], the chest muscle near the shoulder was bothering me more than the previous game," Guerrero said through Jose Mota's translation. "I did a little self-examination and thought it was more swollen than before. The trainers have me going back to L.A. to have it checked.
"It was against the Dodgers at Dodger Stadium. I made a throw to third base and felt it more down by the elbow. But all the pain started going toward the shoulder. It's the first time I've ever had that.
"It doesn't bother me to hit. The concern is because I want to get back to playing right field, and it's not going in the right direction by me touching it. I want to have it looked at now."
Guerrero had eased into Spring Training after undergoing surgery early last October on his right knee.
"No problem with the knee," he said. "It was all taken care of. I just haven't found a rhythm [at the plate]."
Guerrero didn't know if he'd be advised to stay in Southern California for treatment or be allowed to rejoin the team in Minnesota, where the Angels open a three-game weekend series on Friday night.
Scioscia indicated he'd be content with Guerrero remaining in the DH role full time until he's comfortable with throwing the ball.
"We're going to have to be [satisfied with Guerrero in the DH role] for now," Scioscia said. "If it's that, we'll limit him to the bat right now. It's obvious what Vlad brings to our team. The lion's share is what he does with the bat. We'll take it one step at a time.
"If it's something where he can swing the bat all year but won't be able to throw all year, that's a different story. We'll get the prognosis and go from there."
The location of the strain works to Guerrero's advantage as a hitter, Scioscia said.
"I think this is something that would not have as much of an impact on his swing as if it were on the left side -- his front side -- or his lower half," Scioscia said. "Vlad's challenge is keeping his lower half strong with a solid base to get through a baseball.
"He's not quite locked in now."
Guerrero had a single in four at-bats on Wednesday night. It preceded Hunter's two-run homer, briefly giving the Angels a 2-1 lead in a game they lost, 11-3.
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.