I agree with those who say that Fergie's not that great a manager - a tactical one in particular. How easy does it become after 10 years to intergrate 2-3 new (top quality) players into a squad that you've had influence on over so many years? New players will adopt the attitude of current ones. Fergie ruled by fear and tactical adaptation. The squad knew not to cross him and new players will have quickly found that out from others or for themselves (Mark Bosnich). After 10 years, you know the players strengths and weaknesses, what mistakes you've made in the past with the players at your disposal, how other teams set up and play, how the squad is drilled, etc. You then only have to manage on a match-by-match basis depending on who the opposition is.
To win only 2 CL titles in 20 years when you've spent all those years at the top of your domestic league shows that you haven't got what it takes to beat the best in Europe on a consistent basis. Compare the successes of Mourinho, Ancelotti, Guardiola and even Benitez and you'll see what a good tactical manager is capable of.