Wenger insists the Moroccan was no flop.
"People are bit harsh with him as the first six months here he played very well," said Wenger.
"After van Persie came back and went in front of him it became an impossible job because Robin because is world class and it was very difficult for him to compete.
"He lost confidence, he didn't play any more and didn't have exactly the career we expected him to have here.
"But he is a great guy and a great player and I am very happy it is going well for him. I am not surprised that he does well.
"Let's not forget, he had a spell on loan at West Ham first and that did not work as he did not play there.
"Then people came to conclusion that he is not good. For me he is a good player. He is a good in the air, protects the ball well and the way that Palace plays that suits them very well.
"Here the competition was obstructing his career, but I am pleased that he does well as he had a fantastic attitude here."