The Juventus man has been the Red Devils’ top target for much of the summer, but the Old Lady’s reluctance to discuss a deal has caused the saga to drag on.
Louis van Gaal is understood to want the Chilean, but a lack of transfer activity is frustrating him as he looks to build a squad capable of at least a top four finish.
With a deadline of later this week having been set for Vidal, United may well turn their attention back to now Arsenal target Carvalho.
The Sporting Lisbon man was linked with a switch to Old Trafford earlier this year, but the club’s interest cooled and the Gunners have since been mooted as the most likely destination for the rugged midfielder.
He is still seen as an option at United, and could be the man to inject some energy into their central ranks alongside new signing Ander Herrera.
Sporting are believed to be ready to sell due to financial worries, with offers of around £25m liable to be accepted.