"Are you nervous?" Boles asked Millar before the game.
"No, but I know you are," Millar joked.
Boles' move paid off. Millar had no trouble in the field and went 4-for-4, including an RBI double to cap a two-run eighth inning after Johnson left the game, and Florida rallied past the Arizona Diamondbacks 4-3.
Johnson departed with a 3-2 lead after seven innings but did not get his 16th victory. On a humid, 89-degree afternoon he threw 145 pitches, his highest total in two seasons with Arizona, and struck out 11.
Three of Millar's career-high four hits came against Johnson, including a homer leading off the seventh to make the score 3-2.
"If he makes his pitch, you're going to be out," Millar said. "You've got to get that mistake and hit it."
Boles got nine right-handed hitters into the lineup by starting Millar, a reserve who fills in mostly at first base.
"What a great left fielder he is," Boles joked. "Kevin has super confidence in himself, even when he shouldn't."
Luis Gonzalez homered twice, giving him 22 this year, to drive in Arizona's runs. The Diamondbacks lost two of three to Florida and ended a nine-game trip leading the NL West by one-half game over the San Francisco Giants.
The Marlins earned their second consecutive come-from-behind win without left fielder Cliff Floyd and shortstop Alex Gonzalez, who recently went on the disabled list.
Florida's Mark Smith singled to start the eighth against Mike Morgan (4-3). Preston Wilson lined out to shortstop Tony Womack, who threw wildly to first trying to double up Smith, allowing the runner to reach second.
Derrek Lee's two-out single on a 3-2 pitch tied the game, and Millar followed with a run-scoring double.
"I let it get away, end of story," Morgan said. "The key to the inning was the leadoff hitter. He got on, and it led to disaster."
Armando Almanza (3-0) pitched a perfect eighth. Antonio Alfonseca worked the ninth for his 31st save, most in the major leagues.
Johnson allowed seven hits and two runs, one on a wild pitch. He reached double figures in strikeouts for the 16th time this season and the 141st time in his career.
"I battled out there," he said. "It wasn't easy today. I got through some tough innings."
Florida scratched out a run in the first. Luis Castillo led off with a double and stole third. With two out and Wilson at bat, Johnson threw a 2-2 breaking pitch into the dirt, and when the ball rolled behind catcher Kelly Stinnett, Castillo scored easily.
Gonzalez tied it with a leadoff homer in the fourth against Ryan Dempster, then put Arizona ahead 3-1 in the sixth with a two-run homer after Danny Bautista reached on an error by shortstop Dave Berg.
Dempster allowed the Diamonbacks five hits and three runs, two earned, in seven innings.
"You can't be giving them stuff with Johnson on the mound," Dempster said. "You've got to make them earn it."
With a smile, he added, "You wait it out and let Millar do his thing."
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