Hull City's Ryan Mason suffered a head injury and was rushed to hospital during Sunday's Premier League match at Stamford Bridge.
Chelsea won 2-0 with goals from returning Diego Costa and Gary Cahill to leave the visitors in the drop zone, but there will also be concerns for Marco Silva's team regarding their midfielder.
The 25-year-old former Tottenham player and Cahill collided in Hull's box after 14 minutes with both medical teams called to the scene as referee Neil Swarbrick stopped play.
Chelsea's centre-half swiftly returned to his feet and readied himself to play again, while Mason received treatment for around six minutes before departing on a stretcher receiving oxygen. David Meyler replaced him.
The injury led to nine minutes added time at the end of the first half, with Costa giving the Blues the lead with 90 seconds remaining.
Mason was taken by ambulance to St Mary's hospital and was receiving treatment on Sunday evening with the Hull's club doctor in attendance.
After Sunday's match, Hull boss Marco Silva said: "Ryan Mason at this moment is staying in the hospital. I don't know more news about this.
"I hope nothing special, nothing serious, but at this moment the player stays in hospital. The doctor stayed with the player in hospital. I wait for news."
Cahill told Sky Sports: "I tried to get on the end of a cross and we smashed heads. I know he went to hospital and I wish him all the best and I hope it's not a serious injury."
Chelsea head coach Antonio Conte added: "Everyone at Chelsea wants to wish for him the best.
"It was a bad accident with Gary and, I must be honest, after the first half also Gary wasn't really good. He decided to continue the game.
"It was very bad this accident. Everyone at Chelsea, we hope to see him very soon on the pitch and to recover very soon."
Mason started his career at Spurs before leaving White Hart Lane for the KCOM Stadium in a £10m transfer last summer.