United beat rivals Manchester City to the signature of the Dutchman last summer in a deal worth £24m summer.
His 20 goals so far have helped United move seven points clear of City at the top of the table and 18 ahead of Arsenal, leaving Wenger in no doubt as to his importance in deciding the destination of this year's trophy.
"It is painful to see Manchester United so far in front of us," Wenger said.
"You know when you sell Robin van Persie to Manchester United, that (they lead the league) will be the case.
"Robin is one of the best strikers in the world and you know he will score goals for them. He is at the peak of his game.
"He is more patient today than he was five or six years ago, now he can stay up front.
"Sometimes in games you don't see him for 20 minutes, but he knows he will get a chance at some stage and that he will use it."
Wenger rued his own side's now remote chance of glory but said victory over City on Sunday would be a priceless confidence boost.
"A win against City would improve the confidence of the team," he added.
"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."