United have been linked with a big-money move for the midfield enforcer all summer, though Spurs are understandably reluctant to sell.
Reports have claimed that United are long-term admirers of the utility man and that Dier features higher up on Mourinho's wish-list this summer than the likes of Nemanja Matic, but Spurs are refusing to entertain the idea of losing the player.
The 'Daily Mail' reports that United officials sounded out their Tottenham counterparts but were met with a firm answer: he is not for sale.
Though Dier lost his place in midfield last season, playing just 16 games there in all competitions, the 23-year-old showered his versatility by reverting to centre-back and making 32 appearances as a defender under Mauricio Pochettino.
That versatility is a major reason why the Argentine rates the player so highly and why he would be loath to let him go this summer.
On the other hand, it is understood that Spurs are willing to sell defender Kyle Walker, should any interested club meet their £50m valuation of the player.