The Real Madrid winger will miss the trip to Belgrade due to suspension.
But Allen remains positive of the Dragons can topple the Group D leaders.
"We are going to miss him, there is no point hiding from that fact.
"He's key to us and he's a world-class player, but one of the main qualities of this group is that we're all about team work 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's playing it's not as simple as that in international football.
"You learn that and whether its offensively or defensively we have to deal with the Serbians in many different ways."