Defending champion Lille slipped to a second defeat in five games in the French league with a 2-1 loss at Lorient on Friday.
Lille lost just three league matches throughout last season but has only won once in five games so far this campaign and conceded 11 goals.
“We need to do much better,” Lille defender and captain Jose Fonte said. “It's unacceptable. We lacked aggression and a will to win."
Nigeria striker Terem Moffi grabbed the winner in the 86th minute to move Lorient fourth ahead of the rest of the weekend's games.
Lille's defensive lapses resurfaced as forward Armand Lauriente put Lorient ahead after just six minutes. Turkey striker Burak Yilmaz equalized from the penalty spot in the 24th after being tripped.
It was Yilmaz's second goal of the season after his 16 last season proved crucial in the title win.
Central defender Sven Botman almost scored for midtable Lille in the 72nd, but his close-range header from a corner was superbly saved by goalkeeper Paul Nardi.
Moffi then netted the winner after sliding in to meet a cross from the left.
On Saturday, Paris Saint-Germain looks for a fifth straight win when it hosts promoted, Clermont.
PSG fans will have to wait a while longer to see Lionel Messi in action at Parc des Princes. The Argentina star scored a hat trick in World Cup qualifying on Thursday but will be rested.
Struggling Monaco looks for its second win of the campaign when it faces Marseille.