Schmeichel says the pair's characters are too similar.
He told the Daily Mail: "Both of them, in their minds, are right. I think they are very much the same. When you are that powerful, that enthusiastic, have that kind of temperament and that desire to always be the best, you can clash. It is like a drug for them and they have to be the best.
"I thought Roy put it really well in the documentary, when he talks about fear as a motivation. The fear. The fear of not succeeding.
"The fear of letting people down - that's the drive. Sir Alex, I think if he was brave enough to say it, he would say the same but in his eyes, the motivating factor is that you want to be the best."