He has not forgotten how United fluffed their chance of the Treble in 2008 when they surprisingly lost at home to Portsmouth in the quarter-finals and is adamant they won't slip up against the Royals.
"The incentive is there to get to the quarter-finals and I don't think that will be lost on the players, no matter what side I pick," said Sir Alex. "We're taking nothing for granted. With an FA Cup tie, you always have to expect the unexpected.
"It happens so many times, you just don't want to be a casualty or a shock. We've had a few over the years.
"Monday, in many people's eyes, it's Reading, and they will think 'yeah, we've just been to Madrid, it was a great night and Monday will take care of itself'.
"Monday can't take of itself. We have to take care of it, we have to have a performance which respects our opponents and the fact that the FA Cup can produce shocks.
"We've suffered them ourselves and we don't want another one.
"I went to see Reading last week and they're a team that's shown plenty of fighting qualities, as you'd expect being down the bottom part of the league."