Roy Keane left Old Trafford by mutual consent in November 2005 after a prolonged absence with a foot injury, later joining Celtic.
In an interview with the Irish Times, Keane said: "The day I left United, in hindsight, I should have stopped playing really.
"I lost the love of the game that Friday morning. I thought football is cruel, life is cruel.
"It takes two to tango also. I am fully responsible for my own actions but some things are wrong.
"I left on the Friday morning and they told me certain things that day.
"I was told the following week I could not sign for another club. I had been led to believe I could.
"There were certain things I was told at certain meetings that were basic lies. That was part of the exit plans.
"They were thanking me for 11-and-a-half years. I had to remind the manager and chief executive (David Gill) I had been there 12-and-a-half years."