Olivier Giroud will miss both of Arsenal's remaining Champions League group matches after the club made the mistake of not including the striker on their original squad list.
Giroud injured his ankle against Everton in August and was not expected to return until the New Year, after all the group fixtures had taken place.
Clubs can register just 25 players on their 'A List' for the group matches, which must be submitted by September 2 and include eight home-grown names - a list Giroud was not included on.
Arsenal, however, need just one point from their remaining two games to make it through to the the knock-out stages and the 28-year-old Frenchman could be included for the rest of the competition, with the next squad list due on February 3.
Arsene Wenger's side, who are currently five points off top spot, host group leaders Borussia Dortmund on Wednesday night, before finishing with a trip to Turkish outfit Galatasaray, who they beat 4-1 at the Emirates Stadium last month.
Giroud, meanwhile, made his first-team return in Saturday's 2-1 home defeat to Manchester United, coming off the bench in the 77th-minute to grab the late consolation goal.