Gary Cahill said he was "devastated" after Hull City and England midfielder Ryan Mason announced his retirement from football on Tuesday after failing to recover from a fractured skull caused by their accidental clash of heads.
The 26-year-old was involved in a sickening collision of with Chelsea defender Cahill at Stamford Bridge in January 2017 and has not played since.
"Devastated to hear today's news from Ryan," tweeted Cahill. "Competing for a corner is something we've done thousands of times and to see those consequences for a top professional like Ryan is heartbreaking," the England centre-back added.
"Sending all my love to him and his family, and wishing him all the best for the future".
Earlier, Mason announced that, despite his best efforts, he was calling time on his football career.
"I can confirm that following specialist medical advice I am having to retire from playing professional football," Mason said in a statement. "I have worked tirelessly in order to be able to return to the pitch.
"Unfortunately, having taken expert medical advice I now have no option but to retire due to the risks involved given the nature of my injury.
"I am and always will be eternally grateful for the incredible network of people around me who have helped me to recover from what was a life-threatening injury in January 2017."
Former Tottenham Hotspur player Mason was capped once for England, in a 1-1 draw with Italy in Turin in March 2015.
"I am so proud to have come through the ranks to play for my club, the club I love," he said. "To have had the honour of captaining the team fills me with immense pride.
"Finally, representing my country is an honour that nobody can ever take away from me and something that I am so proud to have achieved."