The Premier League side have made just one signing so far - Tyrell Malacia from Feyenoord - and are chasing De Jong among other targets.
But if they don't put all they have into signing the man they want then they risk a rival nipping in to nab him - and they have been there before.
In 1988, Newcastle's Paul Gascoigne was going to Man United before Tottenham hijacked the move.
Gazza even told manager Alex Ferguson he would be joining him and the Old Trafford supremo went on holiday believing United had secured the services one of football's most exciting midfielders.
However, Spurs, under chairman Irving Scholar, swooped in and offered to buy the player's family a house which changed his mind.
"Off I go, and I'm lying by the pool when there is a Tannoy for me to take a call," Fergie explained in an interview with the late David Frost. "It was [chairman Martin Edwards] telling me Tottenham had [signed him instead] by buying Paul's mum and dad a house in the north east."
The promise of a new home wasn't good enough, though, as his dad also asked for a car, given the new house had a garage.
Not only that, Spurs also bought his sister a sunbed!
"She should have got a spray tan and I would have a few more medals," Gazza later joked as he recalled his regret on not joining United who dominated the Premier League in the 1990s.
Needless to say Fergie was not impressed when he returned.
The manager had wanted him almost as soon as he took over at United in 1986.
A year into his reign, he was playing relegation-threatened Newcastle in the First Division.
In midfield for Man United were Norman Whiteside, Bryan Robson and Remi Moses, "all great footballers," Fergie told Frost.
"And he just tore them apart," he said, referring to 20-year-old rising star.
He is the one player Fergie regrets not signing, reflecting that maybe he could have helped him avoid the trappings of living in the capital as a young lad.
"We could have taken London out the road of him. We had Bobby Charlton, a Geordie. We had Bryan Robson, a Geordie.
"We had a structure of players who could have helped him - given him some discipline."
As far as the current crop of stars go, they have begun well despite the lack of reinforcements.
United beat Liverpool 4-0 in a pre-season friendly and, as well as De Jong, are hoping to add Christian Eriksen and Lisandro Martinez to the side, too.
Keep up to date with the latest transfer news with talkSPORT's Live Blog