The Argentine has enjoyed an electric 18 months in Lisbon and looks likely to leave the club this summer with Valencia circling.
But with the player having rejected an initial contract from the Spaniards, United are ready to pounce as Louis van Gaal steps up his transfer plans.
Reports suggest that a £7m offer for the 28-year-old has been lodged and that now the club are waiting for permission to thrash out a deal with the player.
Perez starred for Argentina against LVG’s Holland in their World Cup semi-final clash, helping the South Americans to victory and a spot in the showpiece final.
There is now chance of a swoop being finalised until Perez’s commitments in Brazil are finished, which may allow Chelsea, also are also keen, to sneak in.
United are watching Angel di Maria and Mats Hummels as well.