Derek Langley served as a scout at Old Trafford between 2000 and 2016, and he told Ciaran Kelly of the Manchester Evening News that United could have had De Ligt and De Jong before their senior debuts:
"Players that they turned down? Matthijs de Ligt. We had God knows how many reports on Matthijs de Ligt. Frenkie de Jong and all these players now are sort of around Europe. Dayot Upamecano, who is now at Red Bull Leipzig, was another.
"This was one of the reasons why I had a meeting with Ed Woodward and I told him that I wasn't convinced with the competency of certain people at the club. I told Ed Woodward quite straight: 'If you think I'm going to sit here and just tell you what you want to hear, I'm not that man because I'm going to tell you exactly how I see it.'"
Both players are fixtures in Ajax's starting XI now and have been linked with major transfers to some of Europe's biggest clubs.
De Ligt, 19, made his first-team debut in 2016 and has spent his entire career in Amsterdam, while De Jong, 21, was signed from Willem II in 2015 and became an almost instant hit at the Johan Cruyff Arena.
De Ligt has the potential to be the world-class central defender United have been seeking for so long, while De Jong could be one of the next great central playmakers.
The club would be expected to pay a massive fee for either player in the current market, and Dutch presenter Hans Kraay recently told Fox Sports that Barcelona are stepping up their pursuit of De Ligt (h/t Dutch Football):
However, there's no guarantee that either Netherlands international would have turned into the same talent they are today without the necessary tutelage at Ajax.
The Godenzonen have long shown a knack for nurturing young talent before selling on for major profit. The club could see a huge return on the nominal fee paid out for De Jong three years ago:
The Telegraph's Jason Burt dismissed rumours of De Jong's supposedly agreed move to Paris Saint-Germain, however:
Marcus Rashford, Jesse Lingard, Gerard Pique and Adnan Januzaj are listed as some of the talents Langley helped find and develop during his time at Old Trafford, but De Ligt or De Jong could go on to great things themselves.
De Ligt and De Jong are all the more desirable to United, who have concerns in both central defence and central midfield at present, with the pressure particularly telling on the former, per The Times' Paul Hirst:
Record £89 million signing Paul Pogba hasn't brought about the revolution many were hoping he would, while De Jong—who can also play at centre-back—continues to earn rave reviews.
It seems likely both stars will leave Ajax in 2019, and Langley suggested United's squad would be in far better shape now had the club taken his scouting advice more seriously.