The Old Trafford side is 12 points clear at the top of the Premier League table and faces Real Madrid in the second leg of the Champions League last 16 – a match a number of players will miss out on.
The 71-year-old was reminded of the squad depth that saw Paul Scholes miss a quarterfinal against Inter Milan in 1999.
"In 1999 I left Paul Scholes out of the quarterfinal against Inter Milan yet he was the one who scored the goal that got us through," Sir Alex told MUTV.
"I left him out of the semifinal 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.
"A 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."
Sir Alex is delighted by the competitiveness of his squad, and has been buoyed by how eager every player is to contribute, but warned the squad members set to play in Saturday's league match against Norwich that they must remain diligent.
"It is very difficult to rotate the squad at the moment," the Red Devils boss continued.
"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."