Harden, who was scheduled to start here Thursday in the finale of a three-game series against the Tigers, is being pushed back in the rotation at least two days with a slight right-lat strain.
As a result of Harden's injury, Barry Zito and Joe Blanton have each been moved up one day, meaning Zito will start Thursday and Blanton will start the opener of a four-game series in Baltimore on Friday.
Harden, whose bullpen session Tuesday was cut short after five pitches, is tentatively scheduled to work again Saturday.
"We're hoping that in a couple of days he'll be all right," manager Ken Macha said of Harden. "If he can't start Saturday, it'll be one start he misses, so it's not that serious."
"It's definitely frustrating," Harden said. "I feel so strong everywhere else, and I want to be out there to help the team, but I think they just want to be cautious right now so everything's OK in September."
Asked if Harden would be held back were it late September, athletic trainer Larry Davis said, "This isn't a what-if situation." But when asked if Harden were questionable or doubtful for Saturday, he said, "Right now, I would say that's a question."
Davis said the injury prevents Harden from getting full extension on his pitches, and that pitching through the pain could affects Harden's mechanics and lead to other injuries. Harden will have to throw in the bullpen once before being cleared to pitch, and there has to be a day of rest between that bullpen session and a start, so unless Harden can throw Thursday, he's out for Saturday.
In a related development, Triple-A starter Juan Cruz was on a pitch count in Salt Lake on Tuesday and was lifted after four hitless innings. Should Harden need to miss a turn or two, the A's have the option of plugging Cruz into the rotation.
"I read the game reports [from Sacramento] all the time," Macha said. "[Cruz has] been pitching well."
Another option is moving lefty reliever Joe Kennedy into the rotation, but Cruz has been stretched out as a starter at Sacramento, while Kennedy, a starter himself earlier in the year, has been working in relief since coming to Oakland in a trade over the All-Star break.
Macha has said that he likes having a second lefty in the 'pen, so if Kennedy is put into the rotation, lefty reliever Ron Flores likely will be called up from Sacramento rather than Cruz.
Kendall sits: As is often the case when he doesn't want to say anything he might regret, Jason Kendall was mum on the topic of not starting at catcher on Wednesday.
"I'm just the catcher guy," he said.
Kendall went 0-for-4 on Tuesday and has just nine hits in his last 55 at-bats (.164), but Macha said he's seen some signs of life of late.
"I think he's close to getting his stroke back," Macha said. "So we'll get him a mental day here, and when he comes back, he'll be good."
Adam Melhuse started behind the plate and hit an RBI double to deep left-center field in the second inning.
Dribblers: Scott Hatteberg's strained oblique has improved to the point that Macha said he'd try to get him some at-bats in the next couple of days. Davis said Hatteberg has been cleared to play, but Eric Chavez served as the designated hitter for Wednesday's game. ... Chavez was back with the team after spending Monday and Tuesday with his wife and newborn son, Diego. "It's amazing," Chavez said of fatherhood. "It's so hard being away from them right now. I can't wait to get home." ... As a result of being bumped up a day, Zito will get to face the Yankees in Oakland on the next homestand. "It stinks that Rich is hurting, but I was kind of bummed that I wasn't going to pitch against the Yankees," said Zito, who will face the Angels in Anaheim on the last leg of the road trip. ... Outfielder Jay Payton took advantage of Detriot's in-house haircutter and had his head shaved clean. Zito was next, for a trim and some frosting.
Up next: Zito (11-10, 3.60 ERA) takes on Tigers lefty Mike Maroth (11-11, 4.64 ERA) in the series finale on Thursday. The first pitch at Comerica Park is scheduled for 10:05 a.m. PT.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.