The right-back collided with team-mate Roger Johnson and needed treatment with oxygen on the pitch before being carried off on a stretcher wearing a neckbrace.
The Ivory Coast right-back subsequently spent Saturday night in hospital, before being released on Sunday morning.
Demel was also thought to have suffered a suspected dislocated elbow as well as facial injuries to add to West Ham's lengthy injury list.
Assistant manager Neil McDonald said after the game: "He has taken a bad knock on his head and he has gone to hospital so we are just waiting on the doctor to come back to give us an idea on how he is, but I am sure he is in good hands.
"I think it showed we have got players who put their body on the line which is great and we need to do that more often.
"And if we get a couple of injuries then it gives somebody else the chance to come in and, as long as we stick together and work hard for each other, then we will get our just rewards, just like we did today."