De Jong has been the Red Devils' top target this summer with new boss Erik ten Hag keen to reunite with the former Ajax midfielder.
Contact has been going on between the Red Devils and Barcelona since mid-May, but little progress has been made.
Barca were initially keen to sell and get De Jong off their books amid their financial difficulties.
However, problems emerged with the Catalan club owing the midfielder around £17million in deferred wages, with an agreement over a fee proving difficult to solve.
Barcelona's president, Joan Laporta, then made public statements that they had no intention to sell.
talkSPORT's Alex Crook reports that the Red Devils have a contingency plan should they not land De Jong.
He said: "I think they should probably walk away sooner, to be honest, bearing in mind the first contact between Manchester United and Barcelona was the middle of May - we are almost in the middle of July.
"This is a ridiculous situation. It has been a saga for too long. Either it's going to happen or it's not.
"I know Manchester United have a back-up list. Youri Tielemans is on that list, as I understand it, as is Ruben Neves from Wolves.
"I would call Barcelona's bluff now and say 'either you sell him to us, or we will pay the same money and sign Ruben Neves'. He's a player who is proven in the Premier League."
Leicester star Tielemans has been a target for Premier League rivals Arsenal and could be available for between £30million to £35m due to him being in the final year of his contract.
Neves has been on United's radar for some time and the Wolves man could be available for around £50m.