The Arsenal forward has been ruled out for at least a month as he recovers from a knee injury.
Lacazette picked up the knock during the Gunners’ 1-0 defeat to north London rivals Tottenham on Saturday.
Pierre-Emerick Aubameyang is now Arsenal’s only fit out-and-out centre forward available for their upcoming fixtures.
And Wenger admits Lacazette’s absence is a huge blow for his side.
“Lacazette had knee surgery and he won’t be available for the Europa League game unfortunately,” he told beIN Sports.
“He got a surprising swell of the knee after the game against Tottenham and he needed surgery very quick.
“He’ll be out for the next four to five weeks.
“It’s a massive blow for us and we have to find a solution and nobody could predict that.
“Football does unpredictable things and it’s always a good opportunity for someone to step in and do well.
“Lacazette is down. 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.”
Lacazette was struggling for confidence before his setback.
The 26-year-old has only scored once in his last 13 outings for Arsenal and missed a gilt-edged chance to level proceedings late on against Spurs on Saturday.