The attacking midfielder, who can also play at left-back, was on a three-week trial at Carrington and put pen to paper on a three-year deal at the end of his time with the club.
Boonen told HLN: "It's like in a dream. The training sessions are hard but I don't feel the tiredness when I realise I have signed for Manchester United."
Boonen's brother, Sepp, is a goalkeeper and also interesting United.
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