Wuertz suffered the injury during pregame workouts a couple of days ago. He hasn't pitched since Opening Night on Friday, when he threw one shutout inning with two strikeouts against the Mariners.
It marks the second straight season in which the veteran reliever, who just entered the second year of his two-year deal with the A's, has experienced an early-season injury.
At the start of the 2010 campaign, Wuertz missed the first month because of a bout of shoulder tendinitis. He struggled upon his return, but bounced back midseason, posting a 3.16 ERA and a 1.21 WHIP from the start of July through the end of the season. Overall, he was 2-3 with six saves and a 4.31 ERA.
Wuertz experienced shoulder soreness and was brought along slowly during Spring Training, but he also noted toward the end of camp that he felt better than he had in a long time and was encouraged by his improved velocity.
Manager Bob Geren said he is hopeful Wuertz will be reinstated from the DL when he is eligible on April 17. In the meantime, Geren will have to rely on his starters to go deep into games and make do with a bullpen that has already blown two saves in four games after tallying just 13 in 2010, which tied for fewest in the American League.
Ross returns to the A's for the first time since July 7 of last year, when he was a surprise roster pick out of camp and posted a 1-4 record and 5.49 ERA in 26 games -- including two starts -- before being optioned to Triple-A Sacramento to resume starting duties. He was 2-1 with a 3.55 ERA in six starts with the River Cats, but missed the final two months of the season with a strained right elbow.
The 23-year-old righty entered camp healthy this year and was a fifth-starter candidate alongside Brandon McCarthy, Josh Outman and Bobby Cramer. McCarthy won the battle, and Cramer beat out Ross for the final bullpen spot when the A's decided they wanted Ross to remain a starter.
Ross was scheduled to arrive in Toronto, where the A's are in the middle of a three-game road series, around 6 p.m. ET after a delay in Minneapolis. Geren said the right-hander was available for Wednesday's contest, as was lefty Brian Fuentes, who has been dealing with a blister on his finger.
Still, the A's are playing without closer Andrew Bailey (forearm), who is steadily progressing in his rehab but has yet to return to the mound.
"Everything's on schedule with him," Geren said. "Things couldn't be going better, but we can't rush anything."