"The Giants' All-Star-studded pitching staff will be without its biggest member for at least two weeks, and possibly longer.

Randy Johnson was placed on the 15-day disabled list after an MRI exam revealed a strained rotator cuff.

The Giants, who already had 13 pitchers on the roster, recalled infielder Kevin Frandsen from Triple-A Fresno; Manager Bruce Bochy said left-hander Jonathan Sanchez will start in Johnson's place Friday against the San Diego Padres.

Bochy said Johnson, 45, will begin a strengthening program and "hopefully be back in two to three weeks." But the manager acknowledged that Johnson's timetable will be unpredictable, saying, "I'm sure age becomes a factor at this stage."

Johnson is 8-6 with a 4.81 ERA.

"He was on a pace to win 15 games at least," Bochy said. "This guy's been amazing, what he's done and what he's accomplished. He's disappointed to have to go on the disabled list, because he was feeling great."

Bochy said Johnson felt something in his biceps during batting practice a few days ago. He was in obvious pain after taking a swing against Roy Oswalt in the third inning. He left after giving up two homers and making an errant throw to first base in the fourth."