The 21-year-old has attracted widespread interest following a solid campaign in the Champions League and Bundesliga this season.
Here, Sports Mole looks at the numbers available to the Frenchman should he follow Leipzig teammate Timo Werner in making the switch to Stamford Bridge.
Upamecano has been a Leipzig regular for the past four seasons now and has played with the number five on his back in the last three of those.
The other number used by Upamecano during his time at the Red Bull Arena is '17', which is also taken by a midfielder - Mateo Kovacic having held it since arriving in 2018.
Another common shirt number for centre-backs is 'four', of course, and that is a shirt the former France Under-21s international sported during two seasons with Red Bull Salzburg.
Unsurprisingly, that number is also already in use at Stamford Bridge, with Andreas Christensen - another with an uncertain future - in possession of it since last season.
Another number that Upamecano may be interested in is 'six', and that shirt has been free this season due to Danny Drinkwater's banishment.
Further down the list, '14', '16' and '21' are all currently available.