Sir Alex Ferguson has urged Roy Keane to be more tolerant - if he is to succeed as manager of Sunderland.
Keane's first game in charge is against Derby on Saturday.
Manchester United boss Ferguson told the Mirror: "If you can't deal with the pressure and can't enjoy the job, then you are going to be in trouble.
"Some players think they know what it's like to be a manager but they don't, they can't do it. You have got to be prepared to learn."
Ferguson added: "The most important thing he has to do is find a way of enjoying the job."
Former Republic of Ireland and Celtic midfielder Keane played for 13 years under Ferguson at Old Trafford and was his manager's on-field general.
But that partnership ended when Keane left United in November 2005 - after criticising his team-mates during that season.
However, now the Irishman has joined the managerial fraternity, both men have been exchanging phone calls.
"I've spoken to him several times and I said the same thing to him that I said to my other former players like Steve Bruce," added Ferguson.
"I quite like people talking about this being my last year and wondering if I'm going to go because it means they don't know when I'm going to retire"
"I said you've got to deal with and navigate the problems of today like player power and the media.
"I wished him well and told him he will have to learn to tolerate things in football that he just doesn't agree with.
"And the most important thing he has to do is to find a way of enjoying the job."
The pair are at opposite ends of the managerial spectrum and, while Keane is in his first fortnight in the job, Ferguson is in his 32nd year.
But the Scot, who has been at Old Trafford for almost 20 years and is fast approaching 65 years of age, has no intention of quitting just yet.
"The important thing about working is whether you enjoy it and whether your health is OK," he added.
"Once you get to your sixties you are never guaranteed your health.
"For instance, two years ago I got a pacemaker. That's an indication that you are getting older so you have to prioritise yourself the right way.
"I eat the right things and get a proper rest. I work hard but I go to the gymnasium as much as I can.
"For me, it's two things. Am I enjoying the job? Yes. Is my health OK? I think it's OK as I'm getting the right check-ups and if it wasn't that way I'd change my thinking.
"I quite like people talking about this being my last year and wondering if I'm going to go because it means they don't know when I'm going to retire.
"I don't give them any information about that and let them carry on and when they ask I take pleasure in them not knowing."