"In 1999 I left Paul Scholes out of the quarter-final against Inter yet he was the one who scored the goal that got us through," Ferguson told MUTV.
"I left him out of the semi-final because I couldn't risk him and Roy Keane together as they were both on yellow cards. He accepted it graciously, came on, got booked and missed the final anyway.
"What did he do? He didn't let it affect him. He went on to become one of the greatest players this club has ever had.
"The players sense it could be a big year for them.
"But at this club, in this situation, anything can happen on the run-in. Players can all of a sudden find themselves playing in a final or an important league game because that is the kind of squad I have.
"It is very difficult to rotate the squad at the moment. They all wanted to play against Real Madrid and there were a lot of disappointed players.
"That was fantastic. It is a measure of their own confidence in themselves.
"But we had a chat. I said 'look, don't think for a minute losing a game for Manchester United is a trivial matter'. It is a serious matter."