Keane says he had offers from Real Madrid and Everton before choosing Celtic.
“Leaving United is no stop story. It’s just part of life. You come to the end of working with somebody. I definitely think United treated me badly at the end because I didn’t do anything wrong,” he said.
“When I left United that day my love for the game changed a bit. I was badly hurting. When I went home that day I probably should have retired,” he added.