LONDON - Should Manchester United supporters find themselves weighed down by a Monday morning feeling after this weekend it may be something more than a post-Christmas hangover. For while United fans will be unanimous in wishing Sir Alex Ferguson many happy returns on his 71st birthday, the sentiment will be tinged with a sense of unease.Ferguson cannot go on forever and the fact his birthday coincides with the end of another year somehow increases its significance. The image of a weary old man making way for a new-born baby as 2012 gives way to 2013 hardly applies to the United manager, whose inner rages are burning as brightly as ever if Ferguson’s apoplectic reaction on Boxing Day when Mike Dean allowed Newcastle’s second goal to stand is anything to go by. Yet the inescapable fact is that Old Trafford will be looking for his successor sooner rather than later.The speculation about who follows Fergie has already had a trial run. After Manchester United had won the Champions League in 2008, beating Chelsea on penalties in Moscow, Ferguson announced that he would be stepping down as manager within the next three years. This meant he would have been gone by 2011, but following the 2009 final, which United lost 2-0 to Barcelona in Rome, Ferguson said he would be staying on for as long as his health permitted and that he aimed to win the league championship at least one more time to overtake Liverpool’s total of 18.No sooner had this been achieved than the title left Old Trafford for nouveau riche Manchester City and this season its retrieval has been Ferguson’s main driving force. That and the desire to make up for United’s modest showing in last season’s Champions League.Now that his team have been paired with Real Madrid in the knockout stage of the present tournament, retirement, so far as Ferguson is concerned, will surely be so far from his mind as to be non-existent. These recurring challenges keep him going no matter how tough the going gets. His Aberdeen team beat the Spanish side in the 1983 Cup-Winners’ Cup final and the prospect of taking on José Mourinho’s Real, not to mention Cristiano Ronaldo, will stoke the fires afresh.The knowledge that Ferguson is in the habit of taking wine with Mourinho after matches has prompted the thought that the Portuguese, with his experience of Premier League management at Chelsea, would be a suitable successor to Fergie when he finally steps down. And should United knock Real out of the Champions League the likelihood of Mourinho leaving the Bernabéu will increase since his team are already 16 points behind Barcelona, the runaway leaders in La Liga.Not that everyone at Old Trafford appears to be over-enthused by the thought of Mourinho becoming the United manager. Sir Bobby Charlton’s reported verdict was hardly a ringing endorsement: "He pontificates too much for my liking. He’s a really good coach but that’s as far as I would go.”The reality is that the under-35s among United’s followers will have no clear memory of a time when Ferguson was not in charge and cannot imagine anyone else doing the job. Therein will lie the problem for whoever comes next.It was much that way when Sir Matt Busby retired in 1969 after 24 years as manager. Wilf McGuinness, Frank O’Farrell, Tommy Docherty, Dave Sexton and Ron Atkinson all came and went without making United the force in the league they had once been, and Busby himself even returned for a season. Maybe it will take a similar succession of appointments before United find the right man to replace Ferguson and he will need to be a special one even if he is not actually the Special One.In football all things are possible. Consider the situation the team were in when Ferguson was appointed in November 1986. After 13 matches in the old 22-club First Division, Newcastle were bottom and the three above them, lying 19th, 20th and 21st respectively, were Manchester United, Chelsea and Manchester City. Forty-eight hours after he got the job United lost 2-0 at Oxford United to hardly anyone’s great surprise.Twenty-six years and an Aladdin’s cave of trophies later, and with Old Trafford attendances regularly topping 75,000, it is hard to believe that such a time ever existed. And that is Ferguson’s most lasting mind game.