Celtic's fragile Champions League hopes took a shaking on Thursday when skipper Scott Brown was handed a three-match suspension for his red card against Barcelona earlier this month.
UEFA's control and disciplinary committee decided to give the Scotland midfielder an additional two-match ban on top of the automatic suspension he received for his petulant kick at Neymar during the Hoops' 1-0 defeat at Parkhead, which left them bottom of their group with no points after two matches.
Celtic confirmed they will appeal after describing UEFA's punishment as "excessive".
Brown was always due to miss the visit of Ajax next Tuesday but he will now also sit out the return match in Amsterdam and the home game against AC Milan, unless Celtic are successful in their challenge.
However, should they fail in their bid, UEFA's decision represents a serious blow to Celtic's chances of overcoming defeats in their opening two group games and qualifying for the Champions League knockout stages for a second season running.
In addition to the potential loss of Brown, manager Neil Lennon has major fitness doubts over winger Derk Boerrigter and defender Mikael Lustig for the visit of the Dutch champions, while Adam Matthews and Steven Mouyokolo are long-term absentees.