Geren wouldn't say what the corresponding roster move will be, but he noted that righty Edgar Gonzalez, who has struggled in two starts since being called up from Sacramento, would be available out of the bullpen Monday night.
Along the same line, Geren did not rule out the possibility of Gonzalez's stint with the big league team coming to an end Tuesday.
The skipper did commit to Mazzaro sticking around, though. He'll also start Sunday at home against the Orioles and next Friday in San Francisco against the Giants.
"It definitely isn't a one-shot deal," Geren said of Mazzaro's promotion, which will bring together for the first time in the regular season all three of Oakland's top pitching prospects.
Lefty Brett Anderson, righty Trevor Cahill and Mazzaro, ranked in that order among young A's arms by Baseball America heading into Spring Training, were brought to big league camp this year as long-shots to secure spots in the starting rotation.
Anderson and Cahill made the cut, while Mazzaro, who dominated early on, struggled late before being sent to Minor League camp.
The struggles ended with the start of Sacramento's season. Mazzaro has a 2.38 ERA in 10 games with the River Cats. With the exception of consecutive starts, on April 20 and May 6, in which he allowed a total of 11 earned runs on 20 hits over nine innings, Mazzaro has been lights-out all year. In his other seven games, he's allowed zero or one earned run seven times and two runs once.
His most recent outing came May 27, when he scattered four hits and a walk while striking out six over six shutout innings.
"Vin has been exceptional at Triple-A, and we all feel he's ready to go," Geren said. "It's always good to bring someone into the big leagues who's been on a roll, and Vin's certainly on a roll."
In his past four outings, Mazzaro has allowed one run on nine hits and four walks while striking out 21 over 27 2/3 innings, for an ERA of 0.33.
"I know he's been killing it down there," said A's catcher Kurt Suzuki, who will handle Mazzaro's debut. "You knew it was just a matter of time."
Mazzaro, a third-round pick in the 2005 First-Year Player Draft, is the oldest of Oakland's highly touted trio of youngsters. Cahill and Anderson both are 21. They've been hyped as a huge part of general manager Billy Beane's plan for sustained long-term success since Anderson was acquired in the December 2007 trade that sent Dan Haren to Arizona.
"It's exciting," Geren said of having them all on the game's biggest stage. "You talk about the future, and the future is now."