OAKLAND -- A's left-handed starter Brett Anderson's run of bad luck with injuries has struck again. He has a stress fracture in his right foot that will keep the team's opening day pitcher sidelined for at least a month.

The A's are saying the injury is not related to the sprained right ankle that put him on the disabled list May 1. He pitched in an injury rehabilitation assignment on Sunday but felt additional pain in his foot the next day. That led to the determination of the stress fracture on the inside of his right foot.

The A's medical team will wait a month before reevaluating Anderson, so it is likely he will not be available to pitch until sometime around the All-Star break, if then.

Anderson missed all but seven starts of the 2012 season while recovering from Tommy John surgery, but when he came back, he helped trigger a late-season surge that helped the A's win the A.L. West en route to the playoffs.

Dan Straily has taken Anderson's place in the rotation, but the second-year right-hander has struggled and it's not clear if the club will keep him in the rotation. Sonny Gray has pitched well for Triple-A Sacramento, with a 2.47 ERA.

In an unrelated move, the A's called up left-hander Hideki Okajima from Sacramento and designating right-handed reliever Chris Resop for assignment.

Okajima, a veteran from Japan who had most of his big league success with the Boston Red Sox, has a 2.16 ERA in 14 games for the River Cats. Resop was 1-1 with a 6.00 ERA in 18 games for Oakland.

Resop's fall from grace has been sudden. In his first seven games, he had zero ERA and allowed hitters a .167 batting average. Since then, however, his ERA is 9.26 and batters are hitting .353 against him.