PA:
The Premiership pair [Arsenal and United] were drawn against Porto and Benfica, respectively, in the group stages but if the Portuguese football association are punished for a breach of FIFA rules, they will face new opposition.
Football's world governing body are deciding whether to impose sanctions on Portugal's club and national teams after top-flight side Gil Vicente appealed to a civil court over their relegation, which was imposed for fielding an ineligible player.
Direct judicial or political interference in the running of football is forbidden under FIFA statute and punishments include expelling national teams and clubs from all football.
UEFA director of communications William Gaillard told the News of the World: "The ultimate sanction is that the Portuguese national team and all Portuguese clubs are banned from all competitions by FIFA. Clearly, that would impact on UEFA's competitions, including the Champions League."
FIFA will announce their decision to the Portuguese FA on September 11, two days before United and Arsenal's Champions League campaigns get under way.