Sadlier was originally promised a benefit game by former chairman Theo Paphitis but for one reason or another the game has yet to take place and it was once again under threat until Middlesbrough stepped in and agreed to play The Lions.
Sadlier was just 24 when he was forced to retire from the game in 2003 with a troublesome hip injury but will play some part in the game.
"I am delighted we have finally made this happen," Sads told the club's official website. "It has been almost six years now since I retired and I can't wait for the chance to play once again at The Den and to say a final farewell."