SAN FRANCISCO -- The Giants' hopes of benefiting from a full season of Randy Johnson's services ended Monday when they placed him on the 15-day disabled list with a strained left shoulder.

To replace Johnson on the active roster, the Giants recalled infielder Kevin Frandsen from Triple-A Fresno. Left-hander Jonathan Sanchez, who recently lost his spot on the starting staff to Ryan Sadowski, will rejoin the rotation in Johnson's spot.

What Johnson's misfortune meant for the Giants couldn't be determined, since nobody would specify how much time the 303-game winner will need to strengthen his throwing shoulder. Manager Bruce Bochy offered a two- to three-week estimate for Johnson's absence.

"Hopefully that's all it is and he can go back to the mound whenever he's due," catcher Bengie Molina said. "Hopefully it's nothing serious. That's the first thing we have to think about."

When the Giants signed Johnson to a one-year, $8 million contract last offseason, the Giants probably hoped that he could match his 2008 output for Arizona -- an 11-10 record with a 3.91 ERA in 30 starts. But Johnson appeared on his way to eclipsing that finish by compiling an 8-6 record with a 4.81 ERA in 17 starts for the Giants. Thus, Bochy said, Johnson was "disappointed" when they spoke Monday.

"He wanted to be a big part of this rotation and help us win ballgames," Bochy said.

That still could happen if Johnson makes a timely recovery. But, having thrown a countless number of pitches in his career at the advanced baseball age of 45, he might seem vulnerable to injuries that could require prolonged healing time. Asked if Johnson's age could slow his recovery, Bochy punctuated his response with his best guess for the pitcher's recovery time.

"I'm sure age becomes a factor at this stage, but I can't tell you when he'll be ready," Bochy said. "I put it at two to three weeks."

Johnson hurt his shoulder in Sunday's game against Houston. He initially showed discomfort while swinging and missing on a low-and-outside pitch in the third inning, then left the game while pitching in the fourth inning. Bochy noted that Johnson felt a twinge in his left biceps area three or four days ago while taking batting practice, but nothing serious was suspected.

Asked if a possible source of trouble was the fall Johnson took on his shoulder as he scrambled to make a fielding play during his 300th career victory at Washington, Bochy said, "That wasn't even brought up. Whether it did or not, I can't answer that."

Frandsen's promotion restored the balance between pitchers and position players which had been skewed since June 28, when infielder Rich Aurilia went on the bereavement list and Sadowski replaced him. Since then, the Giants had remained a position player short.

Frandsen, batting .304 with eight home runs and 33 RBIs in 64 games with Fresno, hit .071 in nine games spanning two previous stints this year with the Giants.

In another injury-related development, the Giants announced that infielder Emmanuel Burriss, their Opening Day second baseman, went on Fresno's disabled list with a fractured fifth metatarsal in his left foot. Burriss, who was optioned to Triple-A on June 16, hit .268 in 17 games since joining Fresno.