The Real Madrid star is unavailable due to suspension and even though Allen has acknowledged he will be missed, the midfielder remains optimistic over his side's chances.
"We are going to miss him, there is no point hiding from that fact. He is key to us and he is a world-class player," Allen told the BBC.
"But one of the main qualities of this group is that we are all about teamwork and we are all about resilience.
"I like to think we have always got a 'plan B' even when we have him playing. It can be about relying on him to pull the rabbit out of the hat, but even when he is playing it is not as simple as that in international football.
"You learn that and whether it's offensively or defensively we have to deal with the Serbians in many different ways.
"We see it as a chance to tackle that challenge and we are confident we still have enough in that squad to go out there and get a good result."
Wales sit third in Group D with seven points from five games, trailing leaders Serbia by four points heading into Sunday's encounter.