Hull have announced Ryan Mason has suffered a fractured skull and is in a stable condition in hospital.
The midfielder has undergone surgery at St Mary's Hospital in London following a clash of heads with Gary Cahill during the first half of Sunday's 2-0 Premier League defeat at Chelsea.
Mason is expected to remain in hospital for the next few days.
A Hull statement read: "The club can confirm that Ryan Mason sustained a skull fracture in our fixture at Chelsea this afternoon.
"He was taken to St Mary's Hospital where he has undergone surgery. Ryan is in a stable condition and is expected to remain in hospital for the next few days.
"Everyone at the club would like to express their sincere thanks for the excellent and swift care given to Ryan by both the Accident and Emergency department and Neurosurgery Unit at St Mary's Hospital."
A further update is expected today.
Immediately after the game Hull boss Marco Silva said he was hoping the injury was not serious.
Both medical teams rushed to the scene as referee Neil Swarbrick stopped play.
Cahill quickly returned to his feet and readied himself to return to the action. He completed the game and scored Chelsea's second goal.
Mason was in hospital by then, having received treatment for around six minutes on the field before departing on a stretcher, while receiving oxygen. David Meyler replaced him.
Players from across England and Europe used Twitter to send best wishes to the 25-year-old Mason, while the England team's account posted a message reading: "Stay strong, Ryan" alongside a picture of him earning his sole England cap to date.
The Football Association tweeted: " We'd like to send our best wishes to @RyanMason, who suffered a skull fracture while playing for @HullCity today."
Mason's first professional club, Tottenham, posted: " Everyone at the Club sends their best wishes to @RyanMason for a full and speedy recovery. Get well soon, Ryan."
Mason joined Hull in August last year after being frustrated by a lack of first-team football at Tottenham, where he came through the ranks while also spending time out on loan at Yeovil, Doncaster, Millwall, Lorient and Swindon.
Mason was capped a number of times at youth level by England, with his only senior appearance coming in a friendly against Italy in March 2015, where he set up Andros Townsend's late equaliser in a 1-1 draw.