The Thunder, with point guard Russell Westbrook suspended for picking up his 16th technical of the season, fell at home to Miami 116-107.
Wednesday, both Leonard and Westbrook are expected back as Toronto visits Oklahoma City in the first game of a home-and-home.
The win over the Knicks could've been costly for Toronto, though.
Starting point guard Kyle Lowry, who was playing in his first game after missing two with a sore left ankle, exited the game when he injured his right ankle after Knicks' Mitchell Robinson fell on it.
Lowry said after the game that the ankle was "pretty tender."
"I'm able to walk, a little bit slower, but I'll be all right," Lowry told reporters.
Lowry also called Robinson's play, "a little bit dirty."
"He grabbed me and pulled me down," Lowry said. "I don't think he did it on purpose. I know he didn't think he was going to hurt me."
Lowry's injury isn't thought to be serious but his status for Wednesday's game is unclear.
Since winning seven consecutive games in February, the Raptors are just 6-5. But they still sit comfortably in second place in the Eastern Conference, 2 1/2 games behind Milwaukee.
Before the All-Star break, the Thunder were in third in the Western Conference, staying close with Golden State and Denver near the top. Since the break, though, Oklahoma City is just 5-9 and has fallen to sixth place.
Still, Thunder coach Billy Donovan said he still has faith his team can turn things around.
"I think the guys' confidences are pretty high because they know there's things they can control and do better," Donovan said to the media.
Oklahoma City is hopeful Nerlens Noel, who missed Monday's game with a quad injury, will be able to return against the Raptors.
While the Thunder have played decently without Westbrook -- they're 5-4 without him this season -- they haven't fared as well without Noel (1-4 without him this season). Oklahoma City has given up an average of more than 128 points per game without Noel in the lineup.
The teams will complete their season series Friday in Toronto.
Before the game, the Thunder will retire Nick Collison's number. Collison spent his entire 14-year career with the Seattle/Oklahoma City franchise before retiring after last season.
In addition to Ibaka, who spent his first seven seasons with the Thunder and Collison, longtime teammate Kevin Durant of the Golden State Warriors is expected to be in attendance.
The number retirement is the first for Oklahoma City.
--Field Level Media