And this may not even be Manchester United's biggest error of recent seasons...!
As the Colombian lights up the World Cup with five goals in four games, a report has emerged that United could have signed him for a cut-price fee four years ago, and ignored him to make one of the most confusing deals in recent years instead.
The Guardian report: "The player was available for as little as £5m when he was touted to United, a club who employed a permanent scout in South America and had close ties with the region.
"The agents attempting to bring Rodríguez to Europe received no response from United. A few weeks later, United instead spent £7.4m on Bebé."
While Rodriguez is starring at the World Cup and recently fetched a £39 million fee from Monaco, Bebe's career has floundered since, and he has not made a competitive first team appearances for the Red Devils since the 2010/11 season.
He has however shown slight signs of improvement on loan at Pacos de Ferreira over the past year, and may go onto salvage his reputation, but it likely will not be at United.
Is it the worst decision Sir Alex Ferguson ever made? Probably not, there have been a few over the past decade.
Many would cite picking David Moyes to replace him as more catastrophic, Rodriguez would have taken United forward but at least, Bebe did not take them backwards like the appointment of Moyes did.
Then going back a decade or so, was the transfer splurge on the likes of Eric Djemba-Djemba, Kleberson, Liam Miller and David Bellion, all who went onto show themselves as more Championship than Champions League.
The biggest error of the lot however may involve allowing Paul Pogba to slip through their fingers. United had actually done the hard part, and signed the Frenchman.
Ferguson's reluctance to give him a chance saw him quit the club for Juventus as his contract ran down, and preferred to give Anderson, Tom Cleverley and infamously Rafael chances in central midfield instead.
Pogba is now rated at £60 million and likely would choose to go elsewhere rather than United. Signing James Rodriguez may cost even more.