Brian Brobbey is free to play for Ajax in the Champions League knockouts despite appearing for Leipzig in the group stage according to UEFA.
The rules state that UEFA Champions League, UEFA Europa League, and UEFA Europa Conference League knockout hopefuls are allowed to register a maximum of three new eligible players during the winter.
Since 2018/19, it no longer matters whether those players have already played in the competition for another club, even for another club still in the competition.
Clubs must communicate their desired squad changes by 24:00 CET on Wednesday 2 February at the latest. Once all teams' changes are approved by UEFA in each competition, the full list of ins and outs will be published on UEFA.com.
Brian Brobbey returned to Ajax on loan from Bundesliga club RB Leipzig after struggling to settle in Germany and also not getting enough playing time.