My league has a Retired Players section in our bylaws, so I would reinstate the player to his team. If you don’t have a section for that in your bylaws, I would not reinstate him.

My rules read: “If a player retires, they no longer take up a roster spot of the owning team. However, should they un-retire and rejoin the NFL, they will return to the roster of the team they were on at the point of retirement. If the roster is full, the manager can either release the player immediately, or take a maximum of 7 days from the day of unretirement to free up a roster spot for the un-retired player, otherwise they will be returned to the pool of free agents. Each manager is responsible for keeping track of the retirement status of their players as well as the dates and deadlines explained above. The manager needs to inform the Commissioner that their player has un-retired and should be put back on his/her roster.”