The popular centre forward has been struggling with a hip injury for the past three seasons and made another attempted comeback two weeks ago when he came on as a substitute against Crewe Alexandra. He made another appearance against Stoke City three days later and is believed to have suffered a reaction.
Sadlier, who made his debut in 1997 at the tender age of 16, has had a career blighted by injury and missed out on going to the 2002 World Cup with the Republic of Ireland.