If Johnson has to miss a start, it would mark a rare encounter with adversity for the Giants pitching staff, who placed Matt Cain and Tim Lincecum on the National League All-Star squad before the game. No member of the starting rotation has missed a turn this year due to injury.
Manager Bruce Bochy assured that the Giants wouldn't be hasty with Johnson, 45.
"If there's anything going on, we'll give him some extra [rest]," Bochy said.
Johnson provided no additional insight into his condition.
"It's a strain," he said. "I'm getting an MRI. That's all I have to say."
Fortunately for the Giants, they could use next week's All-Star break to their advantage. Johnson could rest and recover for as many as 15 days -- the length of a disabled-list stint -- and miss just one start.
If the Giants place Johnson on the disabled list, a possible spot starter would be right-hander Joe Martinez, who was scheduled to pitch Sunday for Triple-A Fresno. Martinez made the Opening Day roster as a long reliever but was sidelined with a concussion and three fractures after being struck in his forehead by a line drive off the bat of Mike Cameron in the season's third game.
"Hopefully it's just a minor deal and he bounces back," Giants closer Brian Wilson said of Johnson. "But it's kind of scary, because he has a pivotal role in the starting five."
"You hope he's healthy," utility man Rich Aurilia said. "I don't think there's a reason, if something's bothering him, to rush him back in four or five days."
Johnson breezed through the first three innings, allowing Miguel Tejada's first-inning homer on his third pitch of the game. He was at his fiercely competitive best in the third inning when he escaped a first-and-third, none-out jam without yielding a run.
But after Johnson struck out in the third inning, flailing at a low-and-outside pitch and accidentally flinging his bat away, he immediately grabbed his shoulder in obvious discomfort.
After consulting with the Giants' athletic training staff, Johnson remained in the game for the fourth inning but allowed home runs to Hunter Pence and Jason Michaels. Giants catcher Bengie Molina knew that something was wrong with Johnson.
"He started throwing a lot slower than he usually does," Molina said. "He usually throws 89, 90, 91 [mph]. All of a sudden you see this guy throwing 82."
Johnson's end came shortly after he fielded Roy Oswalt's comebacker with two outs and Jeff Keppinger on first base. Johnson's sidearm throw to first flew wide for a throwing error, enabling Keppinger to score and lengthen Houston's lead to 4-0. Johnson reached for his shoulder after making the wild toss.
Johnson threw one ball to Houston's next batter, Kaz Matsui, then beckoned for Bochy and head athletic trainer Dave Groeschner. After a brief consultation, Groeschner escorted the 303-game winner off the field.
Unfortunately for the Giants, Oswalt (5-4) stuck around. He allowed one hit through seven innings before surrendering San Francisco's lone run on Aurilia's one-out, eighth-inning homer. That was the final inning for Oswalt, who yielded three hits.
"He was tough. He was locating every pitch," said first baseman Travis Ishikawa, who singled leading off the fifth inning for the Giants' first hit. "I saw him three times and didn't see one ball [out of the strike zone]. So he was definitely being aggressive and pounding the zone."
San Francisco reached the season's midpoint with a 44-37 record and possession of the National League Wild Card lead. Though Sunday's outcome denied the Giants a sweep of the three-game series, the weekend as a whole didn't knock them off-stride.
"If we can take two of three every series, we'll put ourselves in good position," Ishikawa said.