The Scot says he should never have left Blackburn Rovers.
He told the Evening Chronicle: "My happiest years in management were at Blackburn and I do think now it was a mistake to leave.
"Newcastle had gone almost 50 years without winning a major trophy in England and like many people before me and after me, I thought I could be the one who could win that trophy.
"I'd won 11 (trophies) in three countries and 25 in my career as a player and a manager, I thought I could be the man to do it.
"If I hadn't left Blackburn then, I would probably still be in management now. My time at Newcastle soured my experience of management.
"I had tremendous support from John Williams (Blackburn's chief executive).
"If he had anything to say, he would say it to my face. That's not always the case at clubs, certainly it wasn't at my next club (Newcastle)."