Zambrano pitched 1 2/3 innings in Monday's 8-2 loss to Atlanta.
Zambrano allowed two inherited runners to score in his first relief appearance Monday night against the Braves. He entered the game in the fifth inning with one out and runners on second and third and the Braves leading 4-1.
LeBlanc is 1-1 with a 1.15 ERA. He was 17-21 with a 4.47 ERA in 52 career starts the past four years with San Diego.
Zambrano is 5-9 with a 4.54 ERA. He gave up five runs, four earned, in five-plus innings of a loss to the Padres on Friday night. Zambrano was strong through five innings before collapsing in the sixth. He has a 6.91 ERA in his last three starts.
Guillen is hoping for better results from Zambrano in relief.
"I want to put him there to see how it works," Guillen said.
The 30-year-old Zambrano is 1-2 with a 5.05 ERA in 37 career appearances as a reliever. He has a career 3.63 ERA as a starter, but posted a career-worst 4.82 ERA with the Cubs last season and has lacked consistency with the Marlins.
"I want to see why he pitches one, two innings and then he lost it," Guillen said.
Guillen said Zambrano was "disappointed" and "embarrassed," but "is taking it the right way."
Guillen said it's possible the Marlins could make a trade before Tuesday's deadline for non-waiver deals.
For now, Guillen said he still expects right-hander Josh Johnson, who has been mentioned in trade speculation, to start in one game of Friday's doubleheader against the Nationals.
"I want J.J. here," Guillen said. "I'm not the GM. I don't want to be the GM.
"Right now, I don't hear anything from them. But if they make a trade I wouldn't be surprised."