Reguilon was on loan last season at Atletico Madrid, and it didn't go well - played very little, partly but not solely due to injury.
Seems not to have impressed Ange in early pre-season, whereas Udogie and Davies did. Reguilon was 'lucky' that ManU had injuries and he's been given a chance at a good level to show what he can do - and really glad to see he is impressing.
He's at ManU on loan for season, and if he's there for the season its up to ManU whether they want to buy him. There's also a break clause in January when ManU or any other club could bid for him. My guess is if there is a bid of £30m or £40m, Spurs will sell to reinvest in the squad, and find another back up Left Back. If no bid is big enough, Spurs will re-assess at the end of the season.