The one-time England worldwide was playing for Hull City during the match after moving to the Tigers following 17 years at the Tottenham Hotspur youth academy, where he joined the club as an eight-year-old.
The talented Hull City man, with one England cap to his name, suffered a head injury against Chelsea back in January 2017.
"I can confirm that following specialist medical advice I am having to retire from playing professional football", Mason wrote in a statement, posted to his official Instagram page.
"Always he is and will be a special player for me because he represent a little bit this change in all the processes when we arrived at Tottenham". I am still very young and I will be passionate about whatever I chose to do and I am confident that I will be a success in it because I know I will throw myself at it and it's going to be exciting.
"Then the first game we played in Seattle, after only a few training sessions, he understood everything". He was talking a lot with us, the coaching staff and [academy manager] John McDermott. "But at the same time, he opens a massive door for his future with his family, with George, his new boy". He was a very successful player, and he will be a very successful person with everything he wants to do. He is 26 but he is an wonderful person and has an fantastic football brain, ' said the Tottenham manager.
Chelsea later tweeted: "Everybody at Chelsea Football Club sends our best wishes to Ryan Mason following his retirement". "Don't worry Ryan, you're going to be a successful person outside the pitch", he says.
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