The Arsenal boss let the Dutchman swap north London for Old Trafford in a £24m deal – and the forward has already netted 20 goals in 26 appearances in all competitions so far this season.
United are seven points clear of defending champions Manchester City at the top of the Premier League table.
And Wenger insists it is no surprise to see the 29-year-old striker have such a significant impact on United’s title challenge.
“You know when you sell Robin van Persie to Manchester United that [they will be top of the league],” said the Arsenal boss.
“Robin is one of the best strikers in the world and you know he will score goals for them.”
Arsenal take on Manchester City in the Premier League at Emirates Stadium on Sunday afternoon.
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