The game, set to take place in on August 1st, is likely to be against either Fulham or Tottenham Hotspur and will be The Lions only pre-season friendly to be played at home.
Sadlier was originally promised a testimonial by former chairman Theo Paphitis after being forced to retired with a hip injury in 2003. However, negotiations have stalled on several occasions - particularly over the choice of opposition, with Celtic originally among the favourites.
The news follows the announcement earlier this week that the club will also honour Sad's former striker partner Neil Harris with a testimonial of his own in 2010.