"This is the first time having it," Mazzaro said of the malady. "But I'm not really concerned. I got it checked out. It's just a mild case of tendinitis. So we'll just see if this rest helps -- get back and maybe get two or three starts to end the season."
Geren remained optimistic that the 22-year-old Mazzaro would be able to finish out this season on the mound.
"If everything goes well, he can throw a bullpen, maybe Sunday," Geren said. "If that goes well, then we can get him back in for a start. But he has to pass each stepping stone to what we feel is acceptable."
In 17 starts this season, Mazzaro is 4-9 with a 5.32 ERA over 91 1/3 innings. Including his 10 games at Triple-A Sacramento, he has thrown 148 innings -- fewer than his total in the Minor Leagues during 2008 -- which is why Mazzaro didn't blame wear and tear for his tendinitis.
"I went 170 innings last year, so I don't think it took that much of a toll on my body," Mazzaro said. "Yeah, it's a long season, but I think I'm used to it, throwing a lot of innings."
While Mazzaro recovers from injury, A's rookie righty Clayton Mortensen will continue to pitch in his place. Mortensen was recalled from Sacramento on Friday to take Mazzaro's turn in the rotation. In his last outing, Mortensen went seven innings, allowing four runs on six hits with three walks and one strikeout in a 6-3 loss to the Mariners.
Mortensen, 24, is scheduled make his second start since being recalled on Friday against the Twins.
"I definitely feel like I'm getting more comfortable up here," said Mortensen, who was acquired from the Cardinals in the July trade for Matt Holliday. "Every day, just go out and work harder and ... it will keep coming."
Since the start of September, Geren has been using a six-man pitching rotation in an effort to save his young starting staff from racking up innings in the final month of the season.
"I've got six in there now," Geren said, "and if Vinnie gets back in, then one of the six will just go to the bullpen."