The 21-year-old joins the Premier League club from the Belgian champions for a fee that surpasses the £20 million the Blades paid for Oli McBurnie in pre-season, although neither club revealed the transfer fee.
They had broken their club transfer record to sign Norway midfielder Sander Berge from Genk. "United have once again smashed the club's transfer record to secure the services of a highly-rated Norwegian international midfielder on a long-term contract," The Blades said in a statement.
Berge has signed a four-and-a-half year contract.