McClellan threw his 2-2 pitch behind Diaz's head. Diaz didn't exactly jump out of the way.
"I was hoping it would hit my head," Diaz said. "I could not believe it didn't hit my head. I guess I lost too much weight."
Diaz, who lost more than 20 pounds in the offseason, reached out to slap McClellan's 3-2 pitch up the middle, driving in Infante and Jones.
Diaz was 0-for-3 with two strikeouts before the hit.
"I've been struggling with strikeouts," he said. "I was just trying to put the ball in play."
McClellan said the walk to Infante was an ominous start.
"In a one-run game, you have to get the leadoff man," McClellan said. "If you don't, eventually it's going to beat you. It didn't go my way. ... I didn't make pitches. I put myself in trouble."
Added St. Louis manager Tony La Russa of McClellan: "The only thing he did wrong was walk the leadoff man."
McClellan said he didn't regret the free pass to Jones, who walked in each of his four plate appearances. The Braves have cleanup hitter Brian McCann and No. 5 hitter Garret Anderson on the disabled list, so the Cardinals issued five walks to Jones in the first two games of the series.
"I had to put him on," McClellan said. "Chipper is one you obviously pitch around."
Kyle Lohse pitched six scoreless innings and left with a 1-0 lead despite fighting an upset stomach.
"I almost threw up in the bullpen," Lohse said. "It's been going around. ... A couple of times I barely got off the field."
Mike Gonzalez struck out the side in the ninth for his third save in four chances. Gonzalez struck out Albert Pujols on three straight called strikes for the second out. Gonzalez threw Pujols three straight balls to begin the at-bat.
"After I saw Matty get up there and do what he did and get those runs, I said there's no chance we can let this game go," Gonzalez said. "There's no chance they're going to touch the plate."
Reyes retired 13 consecutive batters after giving up a single to Lohse with two outs in the second. Reyes, who lost his last seven decisions in 2008 and his first decision this season, avoided becoming the first Braves pitcher to have nine straight losses since Marty Clary in 1990.
"That's the best ballgame I've seen Jo-Jo throw," said Braves manager Bobby Cox, who said the left-hander is following pitching coach Roger McDowell's advice by not trying to overpower every batter.
"He just started getting the idea that he's a pitcher, not a flame-thrower," Cox said. "It's just a work in progress."
Lohse gave up four hits with four walks in six innings to lower his ERA to 1.97.
Yadier Molina had two hits, including a run-scoring single in the first, to extend his hitting streak to 12 games.
The Braves wasted a scoring chance in the third inning when Yunel Escobar hit a one-out single and moved to second on Jones' walk. Escobar ran past third base on Kotchman's groundout and was thrown out trying to return to the base for an inning-ending double play.
The three hits allowed by the Braves set a season low. ... Clary finished 1-10 in 1990 and didn't make another major league appearance.