United have offered £12m plus winger Nani for the Argentinian, while Chelsea sporting director Frank Arnesen has been to see him in action.
But Inter also watched him on Sunday and are now preparing an offer.
Tug of war: Benfica's Angel Di Maria (right) is wanted by several clubs
Benfica president Luis Filipe Vieira is determined to keep him, but says they will not stand in his way if a rival club meets his release clause in his contract.
Vieira said: 'The player will be allowed to leave in the transfer window if another club pays the clause in his contract.
'We need to know what the offer will be and then make a decision on which is the best value for the club.'
United need to move fast for him in case inter, city or chelsea move in for him.
Lolx
No Arsenal Time
gg
come to pool! sell riera!