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Simmons, who played at scrum-half, wing and full-back, made his Tigers debut in the Anglo-Welsh Cup against Gloucester in November 2017, with his first tries coming in the same competition against Wasps the following February.
He signed a new contract in March in the hope of resuming fitness.
Tigers general manager Richard Wilks said the club were “proud” of the way Simmons had coped with his injury and the aftermath.
“As a rugby player, Harry was tremendously gifted. As a person, he is loved by staff and team-mates here at the club, and it’s been difficult for all of us to see his career end this way,” Wilks added.
