Nunez had saved nine straight chances dating back to May 4 before he was victimized by Treanor's first-pitch liner to deep center.
"I've seen him do that. He's one of those guys who comes up big," Marlins manager Fredi Gonzalez said of Treanor. "That pitch, if it's a little bit here or a little bit there, he might have popped it up to short or something. He came up big and drove in two runs."
A Marlin from 2004-08, Treanor received a nice hand from the crowd when he was introduced as the pinch-hitter. He jumped on Nunez's 94-mph fastball and drove it to deep center for his first triple of the season.
Marlins center fielder Cody Ross gave chase on the well-struck ball by his former teammate, who turned into the ironic hero for Texas.
"Cody is a good center fielder. I was hoping he wouldn't stride under it and make a catch," Treanor said. "That would make me feel the opposite than I do now. But that's a big gap. I knew it wasn't going out, but I was hoping to get it over his head."
Nunez has now converted 13 of 16 save opportunities. He was one out away from slamming the door for Josh Johnson, who gave up one run in seven innings while striking out seven.
Johnson was in line to secure his eighth win, but now remains 7-2 on a night he lowered his ERA to 1.86.
"It's tough. [Treanor] came out swinging," Ross said. "He connected really well. Good approach by him to jump on it early. This one stings a little bit."
Had the Marlins held on, they would have once again ridden the coattails of their ace to even their record at 32. Now they are 31-33.
Still, Johnson is building a strong case to return to the All-Star Game. Over his last seven starts, the 26-year-old has allowed three earned runs (four total) in 49 innings.
"It's was one of those things that happens every now and then," Johnson said. "We have to come back tomorrow and get 'em."
Josh Hamilton opened the ninth with a single off Nunez, and he stole second. With two outs, Nunez walked Julio Borbon, bringing up Treanor with runners on first and third.
"That ball just came back over the plate. He was trying to go away on him, and it just came back over the plate," Gonzalez said. "JJ has pitched great, and Leo has been really good. It happens."
In the Marlins' half of the ninth, Neftali Feliz retired the side in order for his 16th save.
The game marked the home debut of rookie right fielder Mike Stanton, who received a standing ovation during his first plate appearance. The 20-year-old, however, was hitless in four at-bats with three strikeouts.
"It was like every other game," said Stanton, trying to keep the game in perspective. "[JJ] pitched great. He gave up four hits, and for us to get the loss is pretty tough. That happens sometimes. We have to go and get them tomorrow."
The Marlins were lined up to collect a victory on a night they managed just three hits. The big blast was a two-run homer by Hanley Ramirez in the sixth inning off C.J. Wilson.
Ramirez's second home run of the month was a significant one. It was a two-run laser to center field in the sixth inning, giving the Marlins a 2-1 lead. Wilson had limited Florida to a bloop single to right by Ronny Paulino in the second inning before Ramirez connected on his 10th dinger of the season.
Gaby Sanchez worked a one-out walk in the sixth, setting up Ramirez's no-doubt homer.
As impressive as Johnson has been the past two months, the Rangers made the Marlins' ace work in the first inning.
Texas used three straight singles in a two-out rally to grab a 1-0 lead. After singles by Ian Kinsler and Vladimir Guerrero put runners on the corners, Hamilton laced an RBI single to center.
Johnson threw 33 pitches in the first inning, with 24 coming after two were out. Yet, the right-hander settled, and exited after 111 pitches and seven innings.
"When you throw 33 pitches in the first inning, it's a little tough to say I'm going to go seven or eight innings," Johnson said. "You've got to keep pushing."