Chelsea had high hopes for Gusto when they signed the then 19-year-old for £30.7 million from Lyon in January 2023, with Graham Potter in charge at the time.
Now 23, Gusto has racked up 144 appearances across all competitions for the Blues, helping them win the UEFA Europa Conference League and the Club World Cup.
For the most part, though, he has had to fight Reece James for minutes whenever the Chelsea captain was fit. That left him struggling to string together a consistent run of games.
James's recent injury, likely to sideline the England man until the new year, appears to have opened the door for Gusto to nail down regular football under new boss Xabi Alonso.
Yet Gusto hasn't started a Premier League match since September, and Josh Acheampong looks to have leapfrogged the France international in the pecking order at Stamford Bridge.
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Widespread reports earlier this week suggested the defender was "assessing his options" and "open" to leaving Chelsea in search of a fresh challenge.
According to England's "Metro", that has put both Manchester clubs on high alert. United and City are said to be keen and monitoring the situation closely, waiting for an opening.
An offer in the region of £75 million could be enough to prise Gusto away from Chelsea, it is claimed.
City's interest goes back some time. Manager Enzo Maresca was understood to be keen on a reunion with the full-back when he took the reins at the Etihad Stadium in the summer.
Gusto has three and a half years left on his current Chelsea contract.
Both United and City face complications over any potential deal in the January winter transfer window.