Chris Coleman is still waiting to learn whether the Liverpool midfielder will be fit to travel to Serbia after a last minute withdrawal from the 2-0 loss to Belgium due to illness
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Illness forced the Liverpool midfielder to withdraw from the squad shortly before the game against Belgium - which Wales lost 2-0 on Friday night.
Allen was one of a number of senior Wales players unavailable for that opening qualifier, with Craig Bellamy, Neil Taylor, Joe Ledley and Jack Collison among others sidelined.
Coleman said the loss of Allen just hours before the game was a bitter pill to swallow and made their game against a star-studded Belgium an ever tougher proposition.
"It was a bitter blow to find out we wouldn't have him," Coleman told Sky Sports News.
"He had a temperature and sickness and we will see in the next 24 to 48 hours how he comes through it.
"With our strongest XI it would have been a tough game - and when you take half a dozen players out, lose Joe Allen eight hours before the game and you go down to 10 men then you've got a mountain to climb."
The 10 men of Wales were then dealt a cruel blow before half time as Manchester City captain Vincent Kompany headed the visitors into the lead. Tottenham's Jan Vertonghen sealed the win with a second-half piledriver from a free-kick.