The Welsh international received lengthy treatment on the pitch after inadvertently colliding with team-mate Brennan Johnson before being stretchered off in the 64th minute.
After Williams was replaced by Cheikhou Kouyate, BT Sport confirmed that the Forest defender was being treated for concussion.
And after the match, Forest boss Steve Cooper confirmed that Williams had been taken to hospital.
Cooper added: “I had a look when he came off and I saw blood.
“So fingers crossed it is not too bad, his health comes first regardless of results.”
Following the incident, the 22-year-old received plenty of positive messages from supportive fans.
One wrote: “Neco! Neco! Neco! Neco! speedy recovery, you’ve had a great season lad.”
And a second tweeted: “Feel better soon, Neco.”
Another added: “So sad for Neco, hope he will be back soon.”
Forest went on to win the action-packed match 3-1 to boost their survival hopes.
Things started badly for Cooper’s men, with Johnson missing an 11th minute penalty.
Facundo Buonanotte then put the Seagulls ahead on 38 minutes, although their lead wouldn’t last long.
Pascal Gross’ own goal levelled things up on the stroke of half-time.
Danilo then put Forest 2-1 up on 68 minutes, with Morgan Gibbs-White’s stoppage time penalty sealing the three points.
Prior to the match, January signing Jonjo Shelvey had been sent home by Cooper after reacting badly to being benched against Liverpool.
SunSport has been told the 31-year-old midfielder “threw his toys out of the pram” and confronted the Forest boss following a team meeting, after learning he had been relegated to the bench for the second match on the spin.
Shelvey’s anger flared when he realised he would not be starting against his former Anfield team-mates and his negative attitude surfaced on the training pitch.
