The reigning AL rookie of the year felt tightness beneath the armpit area of his throwing arm on the third-to-last pitch he threw during his save for the Athletics on Tuesday night against the Detroit Tigers, the fourth outing in five days for the right-hander.
The A's are taking no chances that this could become serious, though Street said he is virtually pain-free now. If it were later in the season he would probably pitch through it.
Street has no history of injury in the area but said he has felt minimal tightness there before from time to time. The pectoralis minor is the smaller of two main muscles in the chest and injuries in that area aren't common for pitchers.
"It's my pec minor and it's minor," said the 22-year-old Street, noting he threw an 89 mph slider and a 96 mph fastball after first feeling the tweak. "Right now it's April and we've got 140 games to play. There's no use blowing it out now. ... I want to be out there for my teammates. That's my job."
Street wasn't sure whether he would be ready to pitch by the weekend against the two-time defending AL West champion Los Angeles Angels.
"The doctors are confident a couple days' rest and he's going to be fine," manager Ken Macha said.
Street had converted 21 straight saves dating to last July 10 before blowing it and taking the loss in Sunday's 5-3 defeat to the Texas Rangers. He allowed career highs of four runs and five hits in two-thirds of an inning.
He then bounced back for his fourth save Tuesday.
"It's better today," he said. "We're being very cautious."
Street, who took over the closer job for the A's last May when Octavio Dotel injured his elbow, went 5-1 with 23 saves in 27 chances and a 1.72 ERA last season -- second among AL closers behind New York's Mariano Rivera, one of the pitchers Street most admires.
When A's trainer Larry Davis asked the pitcher about the level of pain in the area on a 1-to-10 scale, Street said it was a "1½" -- down from 2½ on Wednesday. Street doesn't believe that pitching in the World Baseball Classic for the U.S. team put him behind.
"This is our closer," Davis said. "Early in the year, you will err on the side of caution."