Beckett (8-6) left in the third inning with a strained left side muscle, after allowing three runs. The injury will be reevaluated Wednesday, and it's unknown if Beckett -- who was on the disabled list just a couple weeks ago with a blister problem -- will make Sunday's scheduled start against the Chicago Cubs.
"It was worse out of the stretch than it was out of the windup," said Beckett, who was on the DL last year with a similar injury. "And considering I was out of the ... stretch the whole ... time, that's what happens."
Beckett -- who said his command before the injury was as good as it's been all season -- allowed six hits in two-plus innings, and the Brewers stayed hot once he left. They had 16 hits with five Brewers getting two or more.
Chris Capuano (9-6) pitched into the eighth to win his fourth straight start.
"Sixteen hits tonight. Everybody contributed," said Branyan, who hadn't played since June 1 because of a broken finger. "Capuano threw one heck of a game."
Capuano allowed three runs in the eighth and Derrick Turnbow worked the ninth for his 17th save in 19 chances.
"We're starting to swing the bats a little bit and our pitching has been good," Brewers manager Ned Yost said. "Cappy pitched another great game."
Juan Encarnacion hit a two-run double to highlight Florida's eighth inning. Mike Lowell had three hits and scored a run for the Marlins, who fell a season-high eight games behind Washington in the NL East.
"They hit us, so what can you do?" Marlins manager Jack McKeon said. "Yeah, you don't like it. But that's why you have a 162-game schedule, so you can try to even it up."
Nine of the first 16 Milwaukee batters had hits, and four Brewers -- Damian Miller, Lyle Overbay, Jenkins and Branyan -- each collected two by the end of the fourth inning, as Milwaukee built a 4-0 lead. Miller had two run-scoring singles, one in the second and another in the third, and Capuano settled in nicely.
"I had pretty good command of my pitches," Capuano said, "but you can't do it without guys making great plays."
Capuano needed only eight pitches to get out of the sixth and seventh innings, then was relieved after allowing three straight hits to open the eighth. Damion Easley tripled and scored on Juan Pierre's infield single in the inning, and Encarnacion's hit to left drove home Pierre and Luis Castillo.
Jeff Conine's sacrifice fly in the fifth scored Florida's first run.
Beckett allowed four straight hits and two runs in the second inning, then struck out Capuano, Brady Clark and Rickie Weeks consecutively. He gave up a leadoff single to Overbay in the third, but came out three pitches later -- and Branyan said the Brewers weren't aware he was laboring.
"He can be so dominant," Branyan said. "Either you get him, or you don't."
Beckett walked off the mound and McKeon summoned left-hander Al Leiter from the bullpen. It was the 31st relief appearance of Leiter's career, and his first since Sept. 15, 1993, for the Toronto Blue Jays. Leiter gave up five hits and one earned run in three innings.
"Scale of what, one to 10? Ten. I don't know what to do," Beckett said, when asked about his frustration level with the injuries. "I really don't. It's very frustrating. Nobody seems to have any answers why it keeps happening or anything like that.
- Miller had three hits for the fourth time this season.
- Branyan had three hits for the 17th time in his career.
- The Marlins lost for the eighth time in 46 games when scoring at least four runs.
- The Brewers (40-43) hadn't been within three games of .500 since June 8.
- Carlos Delgado hit into a 5-6-3 double play in the sixth; Milwaukee shortstop J.J. Hardy was shifted to the second-base side for the left-handed hitter, then turned the play after taking a throw from Branyan.
- Capuano has an ERA of 3.28 in his last four starts.