The game was scoreless, the bases were loaded, and Redmond figured Miami might not muster another threat.
His move paid off when pinch-hitter Placido Polanco put the Marlins ahead with an infield single, and they went on to win 2-0 Friday night.
"It's always a tough decision when somebody is pitching as well as Alvarez was," Redmond said. "But that's the best pitching staff in the National League that we're going against, and I wasn't sure we were going to get another chance. We went for it, and it worked out."
Lowly Miami beat the Pirates at their own game, combining dominant pitching with just enough offense. Alvarez earned his first National League victory, and four relievers completed a five-hitter.
The shutout was only the third by the Marlins, but their starters have an ERA of 2.44 since the All-Star break. They're 4-4 over that stretch even though they have totaled only 15 runs.
"It's unreal going in knowing you have a chance even though the bats haven't been there, and knowing one run may be enough," slugger Giancarlo Stanton said. "You can go about the game completely differently."
Stanton provided an insurance run in the seventh with his 12th homer.
Jeff Locke (9-3) allowed two earned runs in 6 2-3 innings and had a career-high nine strikeouts, but he lost for the first time in his past 10 road starts. He walked six, and his ERA rose to 2.15.
"Kind of sloppy," the left-hander said. "I felt really good, but just command wasn't there."
Alvarez (1-1) hit 98 mph on the radar gun, gave up just two hits in six innings, and lowered his ERA to 2.64.
"That was a great performance," Redmond said. "That's the hardest I've seen him throw. You could tell he was pumped up. When you're playing a playoff team, it's good to see guys take it up a notch."
The start was Alvarez's fifth after he sat out the first half of the season because of right shoulder inflammation.
"He just had explosive stuff," the Pirates' Russell Martin said. "His fastball was electric."
Steve Cishek, Miami's fifth pitcher, gave up a pair of two-out singles in the ninth inning but earned his 21st save in 23 chances, including 16 in a row. He retired pinch-hitter Jose Tabata on a groundout with two on to end the game.
Pittsburgh managed only six baserunners, including one erased on a double play, and went 0 for 6 with runners in scoring position. The Pirates struck out six times in the final three innings against Marlins relievers.
The game was scoreless until the sixth, when Locke gave up two walks sandwiched around a single by rookie Jake Marisnick, his first major-league hit. With the bases loaded and one out, Polanco batted for Alvarez and tapped a dribbler toward third for an RBI infield hit.
Third baseman Pedro Alvarez charged to field the ball but made no throw.
"We went for it," Redmond said. "It wasn't pretty, but it worked."
Stanton went 2 for 2 with two walks in four trips to the plate. He led off the seventh inning with a homer off Locke.
"I got the sense he was like, `All right, he hasn't seen anything all day. I'm going to see what he can do with it,'" Stanton said. "You've got to make sure you take advantage of that."
The Marlins had a hit in each of the first five innings, but during that stretch they grounded into a double play and had two runners thrown out. Catcher Martin absorbed a blow when he tagged out Jeff Mathis, who was trying to score from first on Alvarez's two-out double in the second inning.
Martin said he tweaked his left knee on the play, and he left the game in the seventh, but said the injury was minor.
Jordy Mercer extended his hitting streak to a career-best 11 games with a single in the third. That was the Pirates' lone hit until Starling Marte reached on a bunt single with two out in the sixth.
RF Tabata (right forearm) was held out of the starting lineup for the second game in a row. ... Pittsburgh 1B Gaby Sanchez, a Miami native and former Marlin, had 25 friends and relatives at the game in a suite provided by the Marlins. ... RHP Charlie Morton, scheduled to start Saturday, is 2-3 with a 5.06 ERA in six starts against the Marlins.