Nmecha and Roberts joined the Championship club on loan from Manchester City this week and boss Jonathan Woodgate could throw them straight in.
Defender Daniel Ayala (ankle) is definitely out while Darren Randolph (thigh) faces another 10 days on the sidelines.
Marcus Browne (hamstring) will be assessed but defender George Friend (tendon), Ryan Shotton (ankle) and Anfernee Dijksteel (knee) are all missing.
Tottenham will be without England captain Harry Kane.
The striker suffered a hamstring tear on New Year’s Day and is expected to be missing for several weeks.
Tanguy Ndombele (hip), Ben Davies (ankle) and Hugo Lloris (elbow) are also out.
Eric Dier (illness) and Son Heung-min (suspension) are back in the squad.