The north London club triggered a 12-month extension on the Belgian’s deal at the beginning of the year after failing to reach an agreement over a new contract.
By taking up the extension, Spurs activated a £25million release clause which kicks in at the start of the summer transfer window.
United boss Ole Gunnar Solskjaer has already started preparations for a busy summer in the market.
He has made Borussia Dortmund star Jadon Sancho his top target – with a £100m deal mooted for the England starlet.
According to The Sun, United are also keen to add to their defence and have identified the 30-year-old as the ideal player to slot straight into their first-team next season.