The setback is not of great concern, manager Bob Melvin assured, especially considering Anderson is ahead of schedule in his rehab.
"He's going to slow down for a few days here," Melvin said. "It's rare you have no speed bumps in this process, and to this point it's been miraculous he's had nothing."
Anderson, who is in Arizona, most recently pitched one inning in a simulated game, using all of his pitches. He's expected to return to the A's rotation shortly after the All-Star break.