The 29-year-old has been a sensation at league leaders United, who are currently seven points clear at the top of the Premier League table thanks largely to his 16 league goals.
The Gunners' boss sanctioned the Dutchman's £22 million move to Old Trafford despite knowing that his former captain could score plenty of goals for Sir Alex Ferguson's side.
"It is painful to see Manchester United so far in front of us", Wenger told The Sun.
"You know when you sell Robin van Persie to United that will be the case. He is one of the best strikers in the world and you know he will score goals. He got 30 for us in the league last year."
The former Gunners' striker reportedly turned down a more lucrative move to Manchester City in favour of United, but Wenger believes he would have had the same impact at the Etihad Stadium.
He added: "He is at the peak of his game, 29 going 30. He has gone through difficult moments in his career, he knows his game."
Arsenal take on Manchester City on Sunday and Wenger insists the Gunners must win even though the title is now realistically out of sight.
He said: “A win against City would improve the confidence of the team.
“The title looks unlikely but we do not have to give up. We want to fight to come closer and then hope we still have a chance.”