Debuchy scored in the 76th minute after latching onto a Benoit Pedretti cross to put third-placed Lille on 42 points.
Lille goalkeeper Mickael Landreau kept the visitors afloat in the first half with a series of stunning saves and they piled on the pressure after the break, with Belgian prodigy Eden Hazard causing chaos in the Lorient defence.
"Landreau was outstanding and that's why we reached the interval at 0-0," Lille coach Rudi Garcia was quoted as saying by French media.
"In the second half we were solid defensively and managed to score that goal. I had no doubt my team had what it takes to react," he added, referring to last weekend's 5-4 home defeat by Girondins Bordeaux.
Olympique Marseille's hopes of at least claiming the Champions League play-off spot occupied by Lille took a blow when they could only manage a 1-1 home draw against Valenciennes.
Midfielder Alou Diarra put Marseille in front after 16 minutes with a near-post header from a Rod Fanni cross but Valenciennes' Brazilian defender Gil equalised six minutes into the second half, firing home from close range.
Marseille are fourth on 39 points.
Toulouse moved up to seventh on 37 points with a 3-0 win at strugglers Nancy.
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