The midfielder clashed heads with Gary Cahill and was treated for almost 10 minutes on the pitch before being stretchered off.
He was rushed to hospital from Stamford Bridge and will remain under medial supervision for a few days at least.
"The club can confirm that Ryan Mason sustained a skull fracture in our fixture at Chelsea this afternoon," read an official statement.
"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 will be issued tomorrow."
Figures from around the football community have been quick to send their best wishes to Mason, with his former club Tottenham among the first.
Everyone at the Club sends their best wishes to @RyanMason for a full and speedy recovery. Get well soon, Ryan.— Tottenham Hotspur (@SpursOfficial) January 22, 2017
Ex-team-mates from his time at White Hart Lane have also offered their support, with Harry Kane and Gareth Bale wishing the 25-year-old a speedy recovery.
Stay strong @RyanMason— Harry Kane (@HKane) January 22, 2017
Hull colleague David Meyler also took to Twitter to wish a close friend well.