The popular Republic of Ireland striker admitted defeat in his ongoing battle against the hip injury that has plagued him over the past 18 months. After two operations, he made yet another attempted comeback two weeks ago, coming on as a substitute against Crewe and Stoke, but has continued to suffer pain and discomfort.
Speaking to the official club website, Sadlier said "It got to the stage where every day I was getting pain in my back and my hip just doing the most basic of activities,"
"Eventually, I reached the decision that the longer I tried to play, not only was I doing more damage to myself, but the strain was becoming too much - building myself up for each comeback and then being bitterly disappointed when it didn't work out.
"I was told 18 months ago that it was possible that I wouldn't play again and at the time I refused to take that seriously and was convinced that I would.
"Whilst I did manage that, it never lasted for more than two or three games, and it just seemed that after all this time I hadn't made any progress and the symptoms were getting a lot worst.
"I spoke to the new medical people and it just made sense for me to stop now.
"The fact is that this does happen to footballers and unfortunately it has happened to me now."
Sadlier announced his decision to his teammates yesterday morning at Millwall's training ground. He joined Millwall as a trainee in August 1996 and made 119 appearances, scoring 41 goals and won an international cap for the Republic of Irelance as a substitute against Russia in February 2002.