"It's not good," said Harden, who has been out with a strained right lat muscle since Aug. 19. "As soon as I started trying to throw changeups and really extend the arm, I could feel it. Hopefully, it doesn't put me back too far."
Before Harden's setback, A's manager Ken Macha said the club was hoping Harden could return to the rotation on Friday in Boston.
"No way," Harden said after Sunday's loss to the Rangers. "There's no way I can start a game Friday. I don't even know when I'm going to be able to throw, period -- even on the side. We'll see how it is tomorrow, but it seems like every time I try to let it go, it gets hurt again and I'm back where I started.
"It's really frustrating, because it's not a major injury, but it's the kind that just takes a lot of time to get better. And we don't have a lot of time."
Righty Kirk Saarloos, who pitched last Saturday against the Yankees, will get the start Tuesday, followed by Barry Zito on Wednesday and Joe Blanton on Thursday.
Macha had suggested that lefty Joe Kennedy, who made his fourth start as Harden's fill-in Sunday and gave up six runs in 5 1/3 innings, would return to the bullpen if Harden were able to work Friday. Now, it looks like Kennedy will be in the rotation for at least another turn or two.
"Joe's done a good job for us," Macha said. "At the same time, Rich being out has been costly. I don't think anyone can deny that."
Harden has missed 10 starts this year. The A's have won two of those games.
"That's how important the guy is to this team," Macha said.
Dress-up day: The A's annual hazing tradition of making rookies don outlandish outfits for a day continued Sunday, bringing a literal application to one of the players' most frequently used sayings: "Wear it."
Zito, who has been organizing the ritual since he was forced to wear a wedding dress as a rookie, took Dan Haren to a local store Saturday, and the two picked out a variety of elaborate costumes. Among them were matching plug-and-socket outfits for joined-at-the-hip rookies Nick Swisher and Joe Blanton, who live together during the season.
"Oh, they'll be wearing those outfits around the house on off-days," Zito said.
Huston Street, dressed in a skimpy top, short plaid skirt and magenta wig that made him look like a Britney Spears video gone horribly wrong, drew the most attention, but Keiichi Yabu got the most props.
Before the game, Zito said that Yabu, who played 11 seasons in Japan before joining the A's this year, would be spared.
"Dude, he's, like, 38 or something," Zito said. "I'm not going to be the guy who makes a 38-year-old dress like the Village People."
Instead, Yabu's interpreter, Yohei Fukuda, was going to be forced to wear a Geisha girl outfit. Yabu, however, insisted on being treated like every other rookie.
"That's so cool of Yabu," said veteran catcher Jason Kendall. "That's awesome."
Dan Johnson (mustard bottle), Matt Watson (hot dog), Freddie Bynum (reggae woman with coconut bra, dreads and a grass skirt) and Ron Flores (angel in red tights) also were among the fashion guinea pigs, who had to wear the outfits out of the clubhouse in Arlington and keep them on until they were checked into the team hotel in Cleveland.
"We're having the bus stop five blocks away from the hotel, too," Zito said. "So they'll be walking the streets of Cleveland all pimped out."
Bocachica on the way? Bobby Kielty's strained oblique muscle isn't getting much better, and Mark Kotsay's cranky back is starting to stiffen again, so the A's might soon be calling up Hiram Bocachica from Triple-A Sacramento.
A Sacramento source said Sunday that Bocachica has been told he's going to the big leagues when the River Cats' playoff run ends, but it's feasible that Bocachica could join the A's in Cleveland as a function of need.
"All I can go on is what I saw in Spring Training, and he had a great spring," Macha said. "He's got Major League experience and he's a quality outfielder."
Bocachica batted .444 (16-for-36) this spring, but was hit by a pitch and suffered a broken wrist in the final week of Cactus League play. He missed most of the season, but returned to the River Cats in early September and was batting .441 (15-for-34) for them through Saturday.
Up next: Haren (12-10, 4.04 ERA) takes on Indians lefty C.C. Sabathia (13-9, 4.35 ERA) on Monday in the opener of a three-series at Cleveland's Jacobs Field. The first pitch is scheduled for 4:05 p.m. Pacific Time.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.