Ferguson has warned his players not to treat selection for a more mundane domestic fixture as second best.
"It is very difficult to rotate the squad at the moment," he said.
"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."