The Red Devils believed they were closing the gap on Manchester City when they beat their local rivals to the signature of Sanchez in January 2018.
Instead, during Sanchez's stint in Manchester, he scored just five goals in 45 games as City won five major trophies, including back-to-back Premier League titles.
It was a remarkable fall from grace for a player who scored a goal nearly every two games in three-and-a-half seasons for Arsenal and 47 times in three years at Barcelona prior to that.
"I thought he would be a forward who would play all across the line, who scored goals and was tenacious, but he has been an absolute disaster," said former United captain Gary Neville.
Lured in by not having to pay a transfer fee in a swap deal that sent Henrikh Mkhitaryan to Arsenal, United should have been more judicious before making Sanchez, then 29, the Premier League's highest paid player on a four-and-a-half year contract.
Many believe Sanchez's rapid decline can be attributed to burnout from representing his country in major international tournaments for four straight summers between 2014 and 2017.
United's Sanchez set to complete Inter loan move
Yet, even after a rare summer off in 2018, Sanchez completed 90 minutes and scored just once in the Premier League last season.
Injuries contributed to his ill-fated time at Old Trafford and his former Arsenal manager Arsene Wenger pointed to a lack of confidence after a poor start to his United career, from which he never recovered.
"The strength of Alexis Sanchez is to show initiative, to dribble, to take people on," said the Frenchman last season.
"These players are the most vulnerable when they have no confidence any more because their game is based on feeling free to take the initiative.
"He had a high level of physical energy and he has lost that as well."
Sanchez's decision to prioritise the money on offer from United, instead of reuniting with his former Barcelona boss Pep Guardiola at City, proved a misguided career move.
Instead of the creative freedom afforded to attacking players by Guardiola and Wenger, Sanchez found himself in the straightjacket of a United in decline under Jose Mourinho.
"When you play a long time with another team and you play different football in that other team, you have some kind of adaptation that you have to have," said United midfielder Paul Pogba, who himself clashed with Mourinho, in defence of his teammate last season.
But even as others such as Pogba, Marcus Rashford and Anthony Martial prospered under a change of management when Mourinho was eventually replaced by Ole Gunnar Solskjaer in December, there was no upturn in Sanchez's form.
So disastrous has his signing been for United on and off the field, that it even provoked a sea change in the club's recruitment strategy this summer.
In the past, executive vice-chairman Ed Woodward's determination to make a splash in the transfer market would have seen him tempted by star names like Gareth Bale, James Rodriguez or Philippe Coutinho being touted by Real Madrid and Barcelona for a buyer.
Solskjaer counting on Sanchez to make Man Utd impact
Instead, United focused their attention on young, British talent even when it meant paying a premium for Premier League players such as the £80 million Leicester demanded for Harry Maguire or spending £50 million for 21-year-old Aaron Wan-Bissaka from Crystal Palace.
Sanchez's departure to join forces with another United outcast Romelu Lukaku in Milan leaves Solskjaer desperately short up front with Rashford and Martial his only two senior strikers.
Yet, so minimal had his contribution on the field become, his presence will still not be missed.
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