Allen has been out of action since September due to a hamstring injury and has failed to recover in time to face USA this afternoon, leaving Rob Page with a selection headache.
Page already had a major decision to make in goal. Wayne Hennessey has largely been preferred to Danny Ward and Adam Davies and despite the fact the 106-cap stalwart has hardly featured for Nottingham Forest this season, he starts over Ward.
Elsewhere, the Welsh team largely picks itself. Gareth Bale and Aaron Ramsay lead their country out for the historic occasion, with Brennan Johnson and Daniel James providing further support up front.
Defensively, all of Conor Roberts, Ethan Amapdu, Joe Rodon and Ben Davies start as a Welsh team built on togetherness looks for another famous push.
Wales starting XI vs USA (3-4-3): Hennessey; Mepham, Rodon, Davies; Roberts, Wilson, Ramsey, Williams; Johnson, Bale, James