Lille prevailed thanks to a Chris Mavinga own goal and a Dimitri Payet header either side of the interval to move up to eighth in France's Ligue 1 with 37 points from 25 games.
Rennes were eyeing third spot but stayed fifth on 40 points, 11 behind leaders Paris St Germain who travel to Sochaux on Sunday.
Olympique Marseille, who occupy the third Champions League qualifying berth, are three points above Rennes and host Valenciennes on Saturday.
Lille went ahead in the 24th minute when Mavinga deflected a Ronny Rodelin cross into his own net.
Alessandrini, who was called up by France coach Didier Deschamps for a friendly against Germany last week but did not play, was replaced in the 35th with a knee injury after clashing with Aurelien Chedjou.
Alessandrini has been a pivotal player in Rennes' good season, scoring 10 goals and setting up five.
Coach Frederic Antonetti told TV channel BeIN Sport that the midfielder might have torn a cruciate ligament in his knee, which would mean Alessandrini's season could well be over.
Lille added a second on the hour, Payet heading home from Salomon Kalou's perfect cross.
Tenth-placed champions Montpellier host bottom club Nancy on Saturday.