Steve McClaren has paid tribute to Sir Alex Ferguson as the Manchester United manager reaches 20 years in charge of the Old Trafford club on Monday.

The England Head Coach says that the success Ferguson has enjoyed over two decades years in England is down to his constant enthusiasm for the next challenge.

McClaren, who was Ferguson's assistant from 1999 to 2001, saw the true mark of the man after the club had completed the treble of European Cup, Premier League and FA Cup in the summer of 1999.

"I do remember at the end of that season we had won the three trophies and the next day we were having a cup of tea at The Cliff and he was talking about the next season and what needed doing for that,'' McClaren recalled.

"I just thought 'my God, you know that's how he has been so successful'. He achieves one thing and then as they say, moves on to the next mountain and the next mountain is higher and that's what he does.''

Aside from the trophies that Ferguson has achieved, McClaren believes his greatest success has been to keep refreshing the side to ensure they stay at the top.

"It's any manager's dream to build one team and be successful but he has built four or five and along that way has been successful with every one of them and that just proves what a great manager he is,'' he told the League Managers' Association Website.

"When you look at anything you are looking at a legacy and leaving a legacy, and if you are doing that and you want the club to be successful for years to come then it's built on youth and you have to develop your own players.

"He has done that on quite a few fronts. I know when I was there that crop of young players coming through could never be repeated but what they have done is they have become the experienced players.

"The experienced players were characters that taught the kids the principles, the values and how to play the game, how to be at Manchester United and what it takes to play for Manchester United.

"I think that is the key, that you've got great experienced players there who've passed on their education and what they've learnt and their experience to the younger players coming through. So it's a seamless transition really from one team to the next.''

McClaren believes Ferguson's decision to appoint him as his assistant in succession to Brian Kidd was a typically brave, single-minded act by the 64-year-old.

"Obviously it was a huge surprise because at the time Brian Kidd had left and a lot of big names were talked about and I thought it would naturally go to an ex-player because they'd been there so long, someone who he knew,'' the England boss added.

"It was a surprise to everybody when I was chosen, but that just shows the mark of the man really that he's not afraid to go outside of Manchester United, not afraid to look outside at other ideas and other people.''