Fielder and Rivera each hit two home runs, and the struggling Brewers built a big lead before hanging on to beat the San Diego Padres 12-9 on Thursday.
"I've been waiting the whole season to help the team," said Rivera, who'll catch Gallardo on Friday instead of Jason Kendall when the Brewers play Houston. "It feels good that now I'll get a little bit more playing time, my confidence is pretty good right now."
Milwaukee led 9-0 after three, but needed Trevor Hoffman to earn his 26th save.
"You never know how many runs you'll need," Macha said.
The Brewers built their big lead after Fielder hit a 465-foot shot off the Miller Park scoreboard and Braun homered as the Brewers connected a season-high five times.
If Fielder or Braun were happy with their performances, they chose not to talk about them. The players stayed behind in the dining area and didn't leave before the clubhouse closed an hour later despite being asked to be available by the public relations department.
The Brewers, who limped through the past six weeks after entering July with a two-game lead in the NL Central, won a day after sending former All-Star shortstop J.J. Hardy to the minors, designating third baseman Bill Hall for assignment and firing pitching coach Bill Castro.
Manny Parra (8-8) won his fifth consecutive decision despite allowing 13 hits in 5 2/3 innings. He took a nine-run lead into the fifth inning, but allowed three runs on five straight singles and a sacrifice fly.
Kevin Kouzmanoff singled in the first run for his career-high fifth hit and Lopez dropped what would've been the third out to allow another run to score. Everth Cabrera followed with an RBI single, but Mitch Stetter got out of the jam and Hoffman pitched a perfect ninth.
"When you're down 9-0 after three innings and you can get their closer in the game you've done a hell of a job fighting back," Padres manager Bud Black said. "I'm proud of our guys."
Fielder connected in the second and fifth innings, giving him 29 home runs this season with 102 RBI. The Brewers rattled Padres starter Cesar Carrillo (0-1), who allowed every man he put on to score in his short major league debut.
Fielder's towering shot -- tied for the third-longest in the park's nine-year history -- thudded loudly off the scoreboard to the right of the pitch speed indicator, which read 93 mph.
"They're a good hitting team and I fell behind a lot of batters," Carrillo said. "Prince is one of the best hitters in baseball and I threw him a first-pitch fastball and he didn't miss it."
The Brewers scored five more runs in the third.
Carrillo hit Craig Counsell and Braun followed with a homer into the left-field bleachers. Gerut's RBI double made it 7-0, chasing the shaken right-hander, and reliever Luis Perdomo allowed Rivera's two-run shot to make it 9-0.
Fielder's second home run came in the fifth and Parra added an RBI single later in the inning that gave the Brewers an 11-3 lead.
"I thought it was fortunate that Prince came up and hit his second homer after they'd scored three," Macha said. "Unfortunately, I had to go to the bullpen and get Hoffman [after] you score 12 runs and you use almost everybody in your bullpen. But a win is a win, so we'll take it."
- Rivera hit his first two home runs of the season.
- Brewers RHP Jeff Suppan (oblique) is scheduled to make a rehab start at Class A Wisconsin and Brewers RHP Dave Bush (triceps) is scheduled to throw a simulated game on Saturday.