Group F is set to get under way on September 13 with Benfica due to travel to Denmark to face FC Copenhagen on the same night the Scottish champions visit Old Trafford.
However, Portuguese minnows Gil Vicente have taken legal action against their football association (FPF) after being relegated last season for gaining eligibility for a player — Angola international Mateus — at a civil court.
FIFA have given the FPF a deadline of September 14 to resolve the situation or face disciplinary action, which could see a ban being imposed on Portugal’s national team and clubs.
Benfica could be banned from the Champions League, leaving possibly just three teams competing for the two top places and a place in the money-spinning knock-out stages.
A FIFA spokesman said: “There is a possibility of this existing.
“There is an ongoing issue in Portugal with the club Gil Vicente, who are in dispute with the Portuguese Football Association.
“Because of this we have asked the Portuguese Football Association to solve the problem internally or with the sporting governing bodies.
“They have until September 14 to resolve the matter but afterthe deadline we would submitthe case to the emergency committee and they would review the matter.
“A ban is an option and when a federation is banned there are no international matches to be played by that nation and the clubs so that would affect the Champions League.”
UEFA have also admitted Porto, who are in Arsenal’s group,and Sporting Lisbon could be excluded from the Champions League.
European football’s governing body have been looking at which teams could be drafted in as replacements but the timing of FIFA’s action may leave them with no option but to have just three teams in Group F and three in Group G.
A UEFA spokesman said yesterday: “If FIFA decide to ban the Portuguese FA and its clubs then that will obviously have a major impact on our competition.
“But it is only at that point we would have a look to see what else UEFA could do.
“For instance, as one of the options, we would have to decide whether or not to play the group with just three teams."