While there have been more high-profile mistakes in the transfer market, particularly since the retirement of Sir Alex Ferguson, de Gea's dramatic decline since is comparable to that of Mesut Ozil's at Arsenal.
Frankly, the only difference is that United seemingly have the kind of money to spend around that financial Albatross, whereas Arsenal were weighed down by it.
So, the difference is of degree, not of kind.
De Gea - who lost his place to Dean Henderson last season before a strange battle between the two towards the end of the campaign - is thought to be on a wage of around £375k-per-week and still has just over two years to run on his contract.
Indeed, the idea of being able to find a buyer either capable or willing to, not only pay what would presumably be big money in terms of transfer fee, but to offer him wages worth his time to leave United seems far-fetched at best.
Unless a difficult deal can somehow be done (in a post-pandemic market no less), it's hard to see how United sign another elite goalkeeper while paying de Gea's salary too. In short, outside of Henderson, United look unlikely to be able to make a significant improvement on the Spaniard until the summer of 2023.
On Reddit, the excellent @gasipo_opinions further highlighted the decline of the former Atletico Madrid shot-stopper.
Drawing up a graph that looks at his post-shot expected goals averages since the 2011/12 season, his drop in performance has been stark. For those who don't know, that metric looks at how likely a 'keeper is to save a chance based on the quality of it for the attacker, with positive numbers suggesting an above-average ability to stop the ball hitting the back of the net.
After a successful time of things that saw de Dea stop 6, 2 and 4 efforts reasonably expected to have resulted in goals in the 2011/12, 2012/13 and 2013/14 seasons respectively, de Gea started performing at a ludicrously high level.
From the 2014/15 campaign to the player's peak in 2017/18, he stopped 4, 3, 5 and 10 goals, establishing himself as one of the best No.1s in world football.
From there, it got worse.
The 2018/19 season saw him record a rating of only 0, suggesting he was performing at an expected level, before a slight rise to just 1 the year after.
Then came 2020/21.
In a massive swing from his peak, de Gea's post-shot expected goals rating stood at a sorry -2, indicating that he should have conceded at least two fewer times than he actually did. In other words, his performances were below average.
Of course, that doesn't necessarily make him a bad goalkeeper, but his decline has been an alarming one and nothing about his output over the last few seasons suggests things are getting better.
Given how much money United have to pay him, that is a major worry for the club.News Now - Sport News