PHOENIX — The A's were dealt their first injury blow just three days into camp, as it was announced Thursday that right-hander Rich Harden will be shut down for two weeks with a strain under his right arm.

Harden, a candidate to be the A's fifth starter, felt soreness in his right side while throwing off the mound Tuesday, and pitching coach Ron Romanick had him stop, manager Bob Geren said. The training staff monitored his status for two days before the decision to shelf him.

"The exact words from (head trainer) Nick (Paparesta) was we'll wait until he's pain-free," Geren said. "It's estimated it would be two weeks."

The A's have high hopes for this season, but they're getting an early and unpleasant reminder of the injury woes that have sabotaged them for the past four seasons.

It also highlights the gamble they took by acquiring players during the offseason with health concerns. Harden, who pitched for the A's from 2003-08, has been dogged by injuries throughout his career.

He's viewed as a leading candidate to be the fifth starter, as well as a potential reliever. Neither Geren nor Harden would speculate on whether he can still make it back for the start of the regular season.

But two weeks — possibly more — is a large chunk of time for a potential starter to miss during spring training. Harden described the injury as a "strain" underneath his right arm. Geren said it was in the Latissimus dorsi muscle.

Harden said the pain reminds him of an injury that sidelined him early in the 2008 season with Oakland. He returned to make 25 starts and go 10-2 with a 2.07 ERA, so he said he draws encouragement from that. The A's traded him to the Cubs in July of that season.

"I can come back and pitch well," Harden said. "(But) I was excited to get going just because I was feeling so good."

With Harden sidelined indefinitely, the top fifth-starter candidates are left-handers Josh Outman and Bobby Cramer and right-handers Brandon McCarthy and Tyson Ross.

McCarthy, signed to a $1 million deal over the offseason, has a history of shoulder issues, but he pitched well in winter ball and is healthy now. Harden signed a $1.5 million deal with the A's shortly after that.

Outman hasn't pitched in the majors since June 2009 because of ligament replacement surgery in his elbow. Ross was shut down late last season with a sprained elbow but has been healthy for some time now.