Manager Mike Scioscia said an MRI Thursday revealed a strained pectoral muscle. The 34-year-old Guerrero was to fly to Los Angeles Wednesday night to see team orthopedist Dr. Lewis Yocum.
Yocum will determine whether the first player since Lou Gehrig to bat at least .300 with at least 25 home runs in 11 consecutive seasons will rejoin the Angels' road trip in Minnesota this weekend.
Guerrero played in all eight of the Angels' games as the designated hitter before sitting out Thursday night's game at Seattle.
He is batting .250 with just one home run -- his only extra-base hit -- and three RBI. But he said the injury is not affecting his hitting, only his fielding.
"I want to get back to playing right field," he said in the clubhouse through an interpreter. "The way I am swinging the bat has nothing to do with this. ... I just haven't gotten into the swing of things."
He said he first felt discomfort during an exhibition against the Dodgers four days before opening day.
"It progressed to the point that I can't play the field, and I want to take care of it," he said.
Asked if the Angels would be OK using Guerrero indefinitely as a DH only, Scioscia said, "We're going to have to be.
"Obviously [it's about] what Vlad brings to our team, and the lion's share of what he brings is hitting right in the middle of the lineup and swinging the bat," the manager said. "And if we can get that, it goes a long way to building an offense.
"We'll take it one step at a time. If it's something where he's limited to that role, we obviously have enough [outfield] depth to work around that."
Gary Matthews Jr. made his third start of the season in right field on Thursday. Bobby Abreu, the DH Thursday, has started six times in right field. They entered the night with no home runs and six RBI combined.