The German playmaker, who became the club's record signing when he joined from Real Madrid for £42m in September, has been criticised by some for a dip in form in recent weeks, despite Arsenal retaining their place at the top of the Premier League with a 2-0 win over Fulham on Saturday.
"Sometimes people expect him to make the difference alone," manager Wenger told the club's official website. "Sometimes when it doesn't work you expect him to take the ball and put it in the net. He is more a passer than a finisher.
"[Goalscoring] is one of the things he can add to his game for sure. It doesn't need to be criticised, it needs to be encouraged. I think as well that the consistency of the quality of his passing slowly drains the opponent as he passes always the ball when you do not want him to do it. That slowly [allows] us to take over.
"He is 25, he can only get better. The best years are coming for him now."
Arsenal host Coventry City in the the FA Cup fourth round on Friday.