The manager has always admitted that leaving players out of his team is one of the hardest parts of his job and never more so than for glamour games such as United’s match at the Bernabeu.
“I know a lot of players were disappointed at not playing on Wednesday,” Sir Alex told media at his Friday morning press conference at Carrington. “That is to be expected. I would be disappointed if they weren't disappointed.
“But the players who weren't playing were all behind the ones who were. That is good. There’s a good spirit here. That is obvious.”
Madrid travel to Manchester for the second leg on Tuesday 5 March and the United boss says he's certain to field a different side to the one that drew 1-1 in the Bernabeu.
"When you are successful you have to look at the overall squad. It will be a different team who play against Real Madrid when they come here.
"One or two or three who weren't playing on Wednesday could be playing in the next game.
"I have all these international players. Why can't I trust them? I could have played a completely different team on Wednesday and I think we would probably have got the same result."