The Hull City midfielder was injured after knocking heads with Cahill during first half of the 2-0 defeat to the Blues. Nine minutes of first-half stoppage time were needed after Mason stayed down on the pitch at Stamford Bridge.
It has been confirmed by the club that Mason suffered a skull fracture, while it is has also been speculated that the 25-year-old could have had bleeding on his brain.
Mason will remain hospitalized for the “next few days,” according to the Tigers.
“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.
“A further update will be issued tomorrow.”
While Mason was forced off during the match due to the injury, Cahill managed to continue for Chelsea and scored the game’s second goal.