The Dutchman scored 30 league goals last season, helping Arsenal to secure third place and a Champions League spot. But, with only a year left on his contract, the Gunners captain publicly announced in July that he would not be extending his deal, leading the north Londoners to decide to cash in.
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The England forward told talkSPORT: "We're all delighted [that Van Persie is joining]. He's a world-class striker and he's got major ability. He can change games, score goals and create them also. He's only going to help us in the title race."
The arrival of the 29-year-old, who scored 132 goals in 277 appearances for Arsenal, is widely expected to limit Welbeck's first-team opportunities at Old Trafford, despite the youngster having formed a good partnership with Wayne Rooney last season which carried into Euro 2012.
However, the striker remains upbeat about his chances and will be looking to learn from his new team-mate.
"You've got to look at the positive side," he insisted. "I'm going to be playing with a world-class striker. We've got a good core group of strikers now and there's still one or two [coming through]."