The French striker, who was ruled out for up to six weeks on Tuesday after undergoing surgery, scored his last goal for the Gunners in the club's 3-1 defeat to Manchester United in December.
Wenger claimed that the forward's knee, which swelled following Arsenal's 1-0 defeat to Tottenham Hotspur, has been affecting the 26-year-old's performances for some time.
"He got a surprising swell of the knee after the game against Tottenham and he needed surgery very quick," the manager told beIN Sports. "It's a massive blow for us and we have to find a solution. Nobody could predict that.
"Football does unpredictable things and it's always a good opportunity for someone to step in and do well.
"I think he recently must have felt something in his knee. Maybe that's an explanation as to why he wasn't scoring as many goals as he's used to."