Sir Alex Ferguson became United boss 21 years ago today. Here, he recalls the support he received from Sir Matt Busby and talks about some of his toughest decisions...

When I was at Aberdeen I turned down a few offers but when the opportunity came to join Manchester United I couldn't say no. 

I admit Sir Matt Busby was a factor in my decision… when I was a kid I once went to see United play just because I knew Sir Matt would be there. His reputation in Scotland was massive and only enhanced by what he'd done at Old Trafford. He was very welcoming and he genuinely wanted me to do well. I used to really enjoy the chats we used to have. I always remember the advice he gave me about the papers: he told me not to read them!

As anyone in business will tell you, managing change is the most difficult thing.

You form emotional attachments to players that you regard almost as family, people who have served you so well for so long. In the last few years I had to let Nicky Butt and Phil Neville go. They embodied the club's spirit and were United nuts, which made it harder.

Experience doesn't make those decisions any easier. If anything, it makes you a little colder. You know you've been down that road before and you have to do what's best for the club. I always have to put that hat on when it comes to making those business decisions.

Read more - and get your chance to win a new 21 years book signed by Sir Alex - in this week's Red View email newsletter.