The Spain international is widely expected to join the Red Devils with links between the 28-year-old and Louis van Gaal’s side having intensified in recent weeks.
Angel di Maria’s exit was expected to speed up proceedings, with the Barcelona ace a possible replacement, however the deal has dragged on.
And now reports claim that Pedro has put a 72-hour deadline on the deal, with the wide attacker growing frustrated at the lack of progress being made.
Should United have not snapped him up by then, the World Cup winner is prepared to stay at Barca.
Louis van Gaal is understood to have requested that United transfer chief Ed Woodward pay the full release clause-triggering fee of just over £21m for the experienced forward, suggesting that a deal will be reached.