SAN FRANCISCO — Randy Johnson is expected to miss at least two starts as the 300-game winner recovers from a strained shoulder.

The San Francisco Giants placed the left-hander on the 15-day disabled list Monday after he hurt himself on a swing Sunday. Johnson, who last month became the 24th player in major league history to reach 300 career wins, felt something in his arm but initially tried to pitch through it. He then left his start after committing a throwing error in the top of the fourth inning against the Houston Astros.

The 45-year-old Johnson (8-6) underwent an MRI exam Monday that revealed the strain.

“As of now we hope he can pitch when his 15 days are up,” manager Bruce Bochy said before the Giants opened a series against the Florida Marlins. “We’ll continue to evaluate him but also give him time to get his arm back. … This guy’s been amazing, what he’s done and what he’s accomplished. I’m sure age becomes a factor at this stage.”

Johnson, who wasn’t available for comment Monday, had managed to stay healthy this season — a major accomplishment considering his recent medical history. Back problems hampered the 6-foot-10 southpaw, a 303-game winner in his 22nd big league season, in each of the previous two years with the Arizona Diamondbacks.

“He’s been unbelievable,” catcher Bengie Molina said. “He’s been pitching great and he’s been a great influence to the young kids.”

Johnson fell on his shoulder during his 300th win at Washington but Bochy isn’t sure the injury is related. He also took a swing in batting practice several days ago and felt some discomfort in his biceps area, then experienced a similar sensation Sunday.

The skipper met with Johnson after the pitcher arrived at the ballpark Monday.

“He was on a pace to win 15 or so games at least,” Bochy said. “He’s disappointed to have to go on the disabled list because he was feeling great.”

It’s unclear whether this injury could jeopardize Johnson’s career. He has said he would evaluate his future and whether he’ll pitch in 2010 after this season.

Jonathan Sanchez, recently demoted to the bullpen, will rejoin the rotation during Johnson’s absence. Infielder Kevin Frandsen was recalled from Triple-A Fresno to fill the roster spot.