Ferguson was speaking with Charlie Rose of PBS.
"Well the horrible part of the job really is when you have players who have been with you for years the evidence is always on the football field. So when you see a player and you notice that the level has started to dip, there is no point on waiting another two years. You have to act because you will only hurt yourself. He'll not want to recognise that the day has come when he has had his time. To have to say that to a player and make the change is very, very difficult. You can only do that if you have a system where you can fill the gaps and rebuild the team.
"Over the years I have probably built maybe five teams, through the consistency of being there as a manager, and the continuity of the youth system, and the players that you have are not joining last. Even the ones that we buy are not going to last for two or three years. You want them to be lasting six, seven, eight years. So you have to buy at a good age, maybe 22/23 because they have had good experience playing elsewhere and they have got plenty of years left in them. So you can build a continuity of team."
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