Outfielder Milton Bradley's history of health issues came to the fore Monday, as well. He, too, was placed on the DL, giving him three trips to the inactive list during his nine months of regular-season action with the A's.
Harden was replaced on the roster by lefty Dallas Braden, who was called up from Triple-A Sacramento and will make his big-league debut Tuesday in Harden's spot in the starting rotation. Bradley was replaced by outfielder Danny Putnam, who was called up from Double-A Midland to make his big-league debut Monday as Oakland's starting right fielder.
Harden, who was with the team but was unavailable for comment before Oakland opened a two-game set against the host Orioles at Camden Yards, came out of his April 15 start against the Yankees with shoulder tightness and hasn't thrown off a mound since.
An MRI showed irritation in Harden's right shoulder capsule, but the team had been hopeful that an extra few days of rest would allow him to start Tuesday. A's manager Bob Geren conceded on Sunday that Harden would have to miss another turn, but added that he wasn't expecting his oft-injured co-ace to be disabled.
The move on Harden was made retroactive to April 16, so he's eligible to come off the DL on Tuesday, May 1.
"It's just best for the team to be cautious," Geren said. "He's feeling better, so it's still positive."
Bradley, who missed six games from April 11-18 with a strained left hamstring and has been on the DL nine times in his career, aggravated the injury Sunday in Texas, and won't be eligible to play again until May 8.
Geren said he had no regrets about playing Bradley over the weekend and said he was hopeful that Bradley wouldn't miss more than the 15 days.
To create room on the 40-man roster for Braden and Putnam, the A's transferred outfielder Mark Kotsay (back) to the 60-day disabled list and released righty Scott Dunn, who was on the DL at Sacramento. Kotsay is eligible to be activated May 31 but isn't expected back until late June at the earliest.
The new guys: Putnam, the 36th overall pick in the 2004 First-Year Player Draft out of Stanford University, was batting .327 with a .386 on-base percentage for Midland, but he said he was surprised on two fronts Monday.
Putnam didn't expect to be called up from Double-A, and he didn't expect to be starting in his first game in the Majors. He caught a 7 a.m. CT flight out of Texas in the morning and was joined in Baltimore for the happy evening by his parents and his fiancée, Becky.
"Most guys have to go through Triple-A, but obviously, I'm happy with the way it worked out," said Putnam, 24. "[As for starting,] I'd rather get it out of the way quick as possible so I don't spend too much time thinking about it."
Braden, 23, was a 24th-round pick in the 2004 First-Year Player Draft out of Stagg High in Stockton, Calif. Named the A's Organization Pitcher of the Year in 2005 after going a combined 15-5 with a 3.52 ERA in 23 starts at Class A Stockton and Midland, he was limited by shoulder issues to 10 appearances in 2006.
Fully recovered, he was a non-roster invitee to Spring Training this year, and after starting this season at Midland, where he went 1-0 with a 2.25 ERA in two starts, he was promoted to Sacramento and threw six scoreless innings in his only start there.
Geren said Braden earned high praise from A's catcher Jason Kendall during a March 29 exhibition game in Sacramento. Braden was scheduled to pitch for the RiverCats against the A's that night, but he had to switch uniforms before the game and replace Esteban Loaiza as Oakland's starter.
"[Kendall] was really impressed with what he saw," Geren offered. "It's always encouraging to hear that from a veteran."
Dribblers: Sacramento lefty Brad Halsey delivered a bitter diatribe against the organization in Monday's San Francisco Chronicle upon learning he'd been passed over as Harden's replacement in the rotation, but Geren didn't rule out Halsey getting a shot if Braden doesn't work out. ... A's rookie outfielder Travis Buck, who was bothered by a left wrist injury throughout Spring Training, hurt his right wrist while diving for a ball in Texas and was not in the lineup Monday. He said pain in both wrists comes and goes, and he's listed as day-to-day. ... First baseman Dan Johnson (hip) was scheduled to make his first rehab start with Sacramento on Monday night against Las Vegas. ... Members of alternative rock bands Audioslave and Soundgarden were on-field guests of the Orioles during batting practice.
Up next: Braden will get the ball Tuesday opposite Orioles righty Jeremy Guthrie (1-0, 3.38 ERA) in a 12:05 p.m. PT start.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.