Moore sustained a thigh problem in the 2-2 draw with Derby at the weekend while Thomas – who missed the County game – is out with a hamstring strain he picked up in training on Friday.
Boss Roberto Di Matteo may also be without the duo for the clash with QPR at The Hawthorns next Monday.
They join several first-teamers on the sidelines, brining the current total to six players.
Robert Koren is out after having an operation on a hernia problem and James Morrison has a heel injury.
Ishmael Miller and Neil Clement are still ruled out with long-term knee injuries.