The Portuguese brings a buzz to Chelsea mid-season after a poor run of form.
And Potter will hope Felix can flourish under him after a difficult season and limited role under Diego Simeone.
Chelsea face Borussia Dortmund in the last 16 of the Champions League.
The first leg is on 15 February at the Westfalenstadion, while the second leg at Stamford Bridge follows on 7 March.
Felix will be available for the Blues, under Uefa rules, in what represents a major boost to Chelsea’s dwindling hopes of landing silverware.
The Champions League is the last realistic chance at a trophy after exiting the FA Cup in the third round after a heavy defeat to Manchester City, while they currently have a 19-point gap to leaders Arsenal in the Premier League.
Chelsea can “register a maximum of three new eligible players during the winter”, according to Uefa.
The rules changed since the 2018/19 season, meaning it no longer matters whether those players have previously played in a Uefa competition (Champions League, Europa League, Europa Conference League), and it also does not matter if they have played for said team in the same competition as their new team.
Chelsea, and other clubs in Uefa competitions, must communicate their desired squad changes by 24:00 CET on Thursday 2 February at the latest.
Article 46 in Uefa’s rules and regulations concerns Felix and his presence in Chelsea’s Champions League squad for the last 16 tie.
46.01: As of the round of 16, a club may register a maximum of three new eligible players for the remaining matches in the current competition. Such registration must be completed by 2 February 2023 (24.00CET) at the latest. This deadline cannot be extended.
46.02: Any or all of the players from the above quota of three may have been fielded for another club in the qualifying phase, play-offs or group stage of the UEFA Champions League, UEFA Europa League or UEFA Europa Conference League.