Red Devils boss hopes his side does not take Schalke lightly and make the same errors as last year, when it was knocked out of the Champions League before losing to Chelsea
The Red Devils, despite beating Bayern Munich at Old Trafford, were knocked out of the Champions League on aggregate, and only days later they lost to Chelsea in what turned out to be a title decider.
Now United is in a similar situation as they prepare to take on Schalke on Wednesday, with Carlo Ancelotti's men facing the Red Devils on Sunday at Old Trafford.
"These are two massive games, both as important as each other," Sir Alex said, according to The Mirror.
"If you look at the experience of last year, when we lost the league after going out of Europe against Bayern, that has to have bearing on my team selection.
"Sunday’s match against Chelsea is a huge one, which is why I have to pick the right team. There will be some changes, but hopefully the nucleus of the team is still good enough to win us the match."
The Scot added: "It’s what happened last year.
"Exactly the same situation. It’s easy to get complacent in modern life - there are many examples of it.
"I hope we don’t get complacent against Schalke. Our work ethic will be very high in order for us to qualify."