The A's have called up Henry Rodriguez, a 23-year-old righty, to take Mazzaro's place. Rodriguez, who features a mid-90s fastball, has tossed 9 1/3 scoreless innings to start the season.
"He's throwing the ball pretty well," A's manager Bob Geren said. "The reports on him from our Triple-A staff are that he's throwing strikes and obviously velocity has never been a question. It's just his command and his command has been pretty good."
Since the A's have off-days on Thursday and Monday, Geren said the team will go with a four-man rotation for the time being and hope Justin Duchscherer will be healthy by the time Oakland needs a fifth starter. Geren said Duchscherer was scheduled to receive a cortisone shot to his injured left hip at noon on Wednesday.
Rodriguez made three relief appearances for Oakland last season, allowing two runs (one earned) in four innings of action.
Throughout his Minor League career, Rodriguez has always been able to record strikeouts. In 2009, his strikeout-per-nine-innings rate was 15.2.
Rodriguez has also handed out plenty of free passes, though, as his best strikeout-to-walk ratio (2.1) came in 2009. This season, that figure has ballooned to 4.7 in eight appearances.