LeBlanc, 0-5 with a 6.11 ERA in seven starts, was initially scheduled to start on Sunday. Now the left-hander will be going to the bullpen, where he is an option to pitch long relief.
Koehler, who has a 3.32 ERA in six relief appearances, will be making his second MLB start. The right-hander started the 2012 season finale.
"He's going to get an opportunity to go in and make a start, and we'll see how he does," manager Mike Redmond said. "This gives us another lefty in the bullpen, and a guy who can throw multiple innings. We'll flip-flop them.
"Tom has been a starter in Minor Leagues, and he's had success. And it gives us a little flexibility in the bullpen. We've got a guy who can come in and face some lefties. And he's built up to where he can give us multiple innings, if we need him. "
Koehler opened the season at Triple-A New Orleans, where he made three starts and had a 1.06 ERA before being called up.
The Marlins also are flip-flopping Alex Sanabia and Ricky Nolasco in the first two games of their homestand against the Reds, which starts on Tuesday at Marlins Park.
Nolasco will start on Tuesday, while Sanabia, who was hampered by a tight right groin in his last start, will go on Wednesday.