According to the Evening Standard, Reguilon arrived at Hotspur Way on Wednesday night for a medical, which is due to run into Thursday.
The report claims that Mourinho's team are expected to officially confirm a deal before Thursday's Europa League clash with Lokomotiv Plovdiv in Bulgaria.
Madrid will have the chance to re-sign Reguilon for £36m within two years, while Los Blancos will also allegedly have the opportunity to meet any future fee that Spurs agree with another club.
The left-back could make his Tottenham debut against Southampton in the Premier League on Sunday. body check tags ::