SAN DIEGO -- The Giants are holding their breath over much more than Barry Bonds' impending 756th home run. They're also anxious about the unraveling of their pitching staff, which accelerated in Sunday's 5-4 loss to the San Diego Padres.

Starter Noah Lowry departed after three innings with tightness in his left (throwing) forearm, striking an ominous note as San Diego completed its three-game sweep of the Giants. Manager Bruce Bochy indicated that Lowry, who began the day as one of four National League left-handers with 12 victories, probably would miss a start, though more details won't be known until the 26-year-old is examined Monday.

Encouragingly, Lowry has never experienced a serious arm injury, except for a case of left shoulder tendinitis when he pitched at Class A San Jose in 2002. A ligament strength test he underwent Sunday revealed no serious problems.

"There's always some concern, but I don't think it's major," Bochy said. "I think we caught it in time."

Said Lowry, who began feeling discomfort in his final inning, "I'm trying to stay optimistic. It feels like it's just a muscle cramp, in my opinion."

But Lowry sensed more than cramping by the time he escaped a bases-loaded jam in the third by inducing Michael Barrett's foul popup.

"It was pretty sore at that point," Lowry said.

Lowry's premature exit again taxed the Giants' bullpen, which worked overtime as the team posted a 2-4 record on its six-game trip. In those six games, Bochy twice used three relievers, once summoned four relievers and three times called upon five relievers. Left-hander Steve Kline has appeared in seven consecutive games. Vinnie Chulk (4-3), who worked in four of the trip's final five games, allowed a pair of sixth-inning runs that broke a 2-2 tie, although he was victimized by sloppy defense.

The fraying of the pitching staff bodes ill for the Giants, who are six games deep into a stretch of 31 games in 30 days. The Giants may have to demote a position player and recall a reliever from Triple-A Fresno to reinforce the bullpen. Bochy said only that he would discuss the situation with general manager Brian Sabean.

Reflecting the bullpen's desperate, tattered state, Bochy asked Barry Zito to make his first Major League relief appearance after 244 starts. Zito responded by contributing a perfect seventh inning. Zito, who throws twice between starts, insisted that his 17-pitch outing won't hamper him when he starts Tuesday against Washington.

"It's fun to help the team on a day when I didn't come to the yard thinking I would," Zito said, recalling that he made his last relief appearance in the Triple-A playoffs in 1999 as an Oakland A's prospect.

Left-hander Jack Taschner said that Zito's selflessness was appreciated.

"He could have very easily said no," Taschner said. "He's the money guy; he's the franchise guy."

The Giants' other franchise guy, Bonds, received an expected rest one day after hitting his 755th career homer to tie Hank Aaron atop the all-time list. Bochy said that he was prepared to use Bonds as a pinch-hitter in the ninth inning had a clutch situation materialized, but the game ended with the Giants two plate appearances away from the pitcher's spot.

Earlier in the ninth, pinch-hitter Bengie Molina's two-run homer off Trevor Hoffman lifted the Giants' hopes. Mark Sweeney followed with a single, but Hoffman recovered as Dave Roberts flied out and Randy Winn struck out.

In a direct correlation, the Giants lost each game of this series by one run while batting .133 (4-for-30) with runners in scoring position.

"We had problems getting a timely hit the whole series," Bochy said.