The French striker has played just 127 minutes for the Blues so far this season, but made the most of his time on the pitch here with an injury-time finish after Timo Werner's effort had been blocked.
Just six minutes earlier, Rennes looked to have denied Chelsea when Serhou Guirassy headed home from a corner to cancel out Callum Hudson-Odoi's opener.
Frank Lampard's side dominated the first half and could easily have led by more than just Hudson-Odoi's calm finish after he had been sent clear by a pinpoint pass from Mason Mount.
