Mata, despite being a fan favourite at Stamford Bridge, was ostracised by Jose Mourinho - now the Spaniard's boss at Old Trafford - and the Portuguese might this time be the benefactor of Willian's comparable scenario.
Once one of the first names on the Blues' teamsheet, the Brazilian's status as a starter has been put under threat by Pedro's best form since his time at Barcelona, while Eden Hazard's displays have made him impossible to dislodge.
But while losing Mata to a desperate Man Utd side led by David Moyes - coupled with a tidy windfall of £37 million - seemed fairly innocuous at the time, selling Willian to a team that next year will surely represent title rivals would be altogether more dangerous.
Despite his paltry tally of starts (only 13 in the Premier League), the Brazilian has recorded one of his most productive seasons to date. Contributing 11 goals in 19 starts across all competitions, the attacker's importance was perhaps highlighted best by two hugely important FA Cup goals against Tottenham.
Additionally, a double against Stoke City was vital in a game where Chelsea were struggling to find a way through, while a stunning counter-attacking goal in the 3-1 away win at Manchester City showed true class on top of grit.
Throughout his time at Chelsea, Willian has, in stark contrast, shown a trait Manchester United are currently sorely missing: ruthlessness. Whether he has been on from the start or emerged from the bench, he has made the most of his playing time.
Yet, despite his performances, he rarely starts. Why? Well, Antonio Conte arrived to a bloated squad used to playing in the Champions League, but Chelsea failed to qualify for Europe entirely after last season's tenth-placed finish, leaving several players surplus to requirements.
Averse to squad rotation at Juventus, Conte has maintained the same stance at Stamford Bridge - and to great effect. The Italian gets the best results when he uses his tried and tested starting XI, benefiting from cohesion through a lack of injuries to his core players. As such, Willian and Cesc Fabregas have had to play second fiddle to Pedro and Nemanja Matic.
Now officially qualified for the Champions League, however, the Blues will need all of their best players ahead of a busy fixture schedule next season.
Willian was Chelsea's Player of the Season last year and, without him, the picture could have been even more bleak as Conte arrived to fix Mourinho's mess in his first role away from Italy.
He started each of the Blues' first six games of the season under the new boss but then tragically lost his mother whilst on international duty. Conte and his staff then withheld Willian in an astute move to protect their talent.
Pedro's run of form since then surprised many and Willian has been battling to regain his spot ever since. He has been a flat-track bully in Chelsea's cup run and devastating off the bench elsewhere, but hasn't quite convinced Conte to tinker with a team that is already successful.
If Manchester United can somehow convince Willian into a move, then Chelsea will face a stronger title rival next season - a team with an experienced Premier League star who is hungry for more glory.
The Blues will resist offers - and that's the right position to take - but Conte may also need to utilise the best of his man-management powers to keep Willian from getting unsettled by the lure of Mourinho.