Nottingham Forest midfielder Morgan Gibbs-White and teenage Chelsea winger Jesse Derry both had to be withdrawn with head injuries during Forest's 3-1 win at Stamford Bridge on Monday.
Derry, 18, had to be stretchered off in the first half after he collided with Forest defender Zach Abbott. Chelsea later confirmed that he had been taken to hospital as a precaution but was conscious and talking.
While there was less immediate concern over the health of Gibbs-White after a collision with Robert Sánchez, the incident has raised fears that he could miss Forest's crucial Europa League semifinal second leg against Aston Villa on Thursday.
The incident involving Derry, who was making his first start for the club, occurred when he jumped for the ball in the Forest box and clashed heads with Abbott after the Chelsea player got to the ball first. It resulted in a penalty to the home side.
Both players went to ground, with teammates and referee Anthony Taylor quick to signal to the benches that both needed attention from medical staff.
Derry remained lying down for around 10 minutes, during which time he appeared to receive oxygen from Chelsea staff.
When the youngster was eventually stretchered off, he received applause from both the Chelsea and Forest fans. He was replaced by Liam Delap.
"Jesse Derry has been taken to hospital as a precaution following his first-half substitution during today's Premier League game against Nottingham Forest," Chelsea said in a statement after the game.
"Jesse is conscious, talking and undergoing precautionary checks. We wish him a speedy recovery and thank the medical staff for their swift response."
Chelsea coach Calum McFarlane, speaking after the game, said he was please with Derry's performance during the game but admitted his health was the most important aspect.
"I think there are more important things than that [his performance] right now. We're hearing positive things from Jesse and Rob, and I hope the Forest players are okay as well.
"But yeah, I'm pleased for Jesse's performance. I thought he performed well while he was on the pitch, but there's probably more important things to think about right now."
Forest also used a concussion substitute to take off Abbott, but the defender was able to walk off the pitch himself. Neco Williams took his place.
The resulting penalty, taken by Cole Palmer, was saved by Matz Sels, meaning that Forest went into half-time 2-0 up.
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In the second half, Gibbs-White and Sanchez collided as they raced onto a loose ball.
Both players were withdrawn as concussion substitutes but, despite receiving bandages to their heads, were able to walk off the pitch unassisted.
Forest are 1-0 up in their Europa League tie following Chris Wood's penalty at the City Ground in the first game.
