Harden was bumped from his scheduled start Tuesday at Toronto because of a mild subscapularis strain, more commonly known as soreness under his right shoulder blade.
The 26-year-old right-hander figures he suffered the injury after making a pair of 18-hour flights to and from Japan for Oakland's season-opening series against Boston. He first felt sore in a side session after the A's got back from the other side of the Pacific.
The injury didn't keep Harden from starting against Boston on April 2, but he left after five innings because he was struggling with his velocity.
Harden tried throwing in the outfield at Toronto's Rogers Centre before Wednesday's game but still felt sore.
"I don't know if it's too likely that I'll be starting Saturday, but we'll see," he said. "It's the one spot on my body that I don't take any chances with."
Manager Bob Geren plans to wait and see before deciding whether Harden is healthy enough to face the Indians.
"I'll know more about him tomorrow," Geren said. "He played catch today and we'll see how it goes tomorrow."
The hard-throwing Harden is 1-0 with an 0.82 ERA this season. His 15 strikeouts in 11 innings are second only to Boston's Daisuke Matsuzaka.
Injuries limited Harden to 13 starts in the past two seasons and 19 the year before that. He's been on the disabled list five times in that span.
The A's don't want to take any chances with Harden or righty Justin Duchscherer, who landed on the 15-day DL Tuesday with a strained right biceps muscle. Duchscherer, who left his start against Cleveland last Friday with arm pain, is scheduled to play catch Thursday.
John Arthur is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.