Both pitchers visited Dr. Douglas Freedberg this week and underwent MRIs that revealed the elbow injuries.
Bailey, expected to be down five to seven days, was diagnosed with lateral epicondylitis, a condition usually caused by overuse that involves soreness on the outside of the upper arm near the elbow.
"It's just a little discomfort," the reigning American League Rookie of the Year said. "I felt something after my last outing [Sunday], so I figured I should have it checked out. I'll just be doing some therapy and give it some rest.
"Last year was probably the heaviest workload on my arm, so this is probably just from kicking it back up and my arm trying to get used to it again."
Meanwhile, Breslow won't be throwing for three to four days as he recovers from medial tendinitis, which is giving the southpaw pain near the elbow joint. The 29-year-old mentioned he felt "relieved" after hearing the results, knowing that the setback is "a little disconcerting, but nothing serious."
Both are still expected to be ready by Opening Day, as manager Bob Geren noted that "everything sounded encouraging."
Bailey and Breslow join right-handers Joey Devine and Michael Wuertz on the team's very own bullpen injured reserve list. Devine, experiencing tendinitis in his surgically repaired right arm, was slated to play catch Wednesday but will instead give his arm one more day of rest before picking up a ball.
"I'll just be playing light catch up to about 80 feet," Devine said. "Free and easy -- that's the goal."
Along the same lines, Wuertz was expected to join Justin Duchscherer in throwing in a Minor League intrasquad game Wednesday. Instead, he'll give his shoulder yet another day off while Geren said there's a "chance for tomorrow" for him to take the mound.
Aside from Devine, Wuertz is the only A's reliever yet to appear in a game. However, just like Bailey and Breslow, Geren said Wuertz "feels he'll be ready" by the start of the season.
"His shoulder wasn't bouncing back like he wanted it to," the A's skipper said, "but he should get in a game soon."
Bailey, Breslow and Wuertz served as key members of a 2009 bullpen that combined for the American League's lowest ERA (3.46). The three relievers, who all appeared in at least 60 games, tallied a total of 19 wins to go along with an average 2.36 ERA.
Jane Lee is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.