Wenger admitted: "The error of Vermaelen was unwelcome. The big defenders don't make those errors."
Ironically, Vermaelen has taken over the Arsenal captaincy from Van Persie and Wenger believes the new role has been a factor in the player's poor form.
The manager added: "You put more pressure on yourself when you are the captain.
"You want to be the best player in the team. But you cannot be a defender and not make a mistake.
"He is a guy who is completely involved in the game, he breathes his football from morning until night. He thinks about it and wants to do well.
"Physically he is outstanding. I think what has happened to him in recent weeks has affected his confidence. Overall, physically, he has no problem."