"I think Roy can be anything he wants to be. The sky's the limit for him," said O'Neill. "Can you imagine the wealth of experience Roy would have picked up from spending all those years and years playing under Brian Clough and Sir Alex Ferguson?
"He couldn't have had two better mentors. All he's had to do is take his own personality into it and be his own man."
He added: "I've had very, very few conversations with Roy, so mine is very much an outside viewpoint. But if he still has the same drive, determination and ambition he had as a player, there's nothing to stop him reaching the very top as a manager.
"There's no question he was one of the greats of the last 10-15 years and acknowledged by just about everyone as one of the top five Premiership players of that time. He stepped in at Sunderland with no managerial experience and turned results around and won promotion from the Championship at the first attempt.
"He's had a supportive board but with his winning mentality and refusal to accept second-best, I have no doubts he can take Sunderland on to the level he would at least be satisfied with for the time being."