United go into the Old Trafford encounter two goals ahead thanks to their efforts in Germany last week. Wayne Rooney and Rio Ferdinand are amongst those who look set to miss out, but Ferguson insists his team will have their minds fully focussed on a Wembley date with Barcelona on May 28.
"It is always dangerous in football to assume you have cracked it; that you have found a sliver bullet or discovered some secret formula for success," said Ferguson. "We live in exciting times for Manchester United and while I want both players and supporters to enjoy every minute of it, we must take nothing for granted."
He added: "It is very easy to get complacent in modern life. There are many examples of it. But Manchester United don't regard it as a formality and it certainly will not be."
The 2-0 lead over Schalke offers Ferguson valuable breathing space given the Chelsea juggernaut will roll into town on Sunday for a game that is expected to determine the destination of the championship.
Ferguson had hoped to claim something from last weekend's trip to Arsenal, but defeat - plus the memory of how United suffered at Chelsea's hands right in the middle of two huge tussles with Bayern Munich - has led Ferguson to no other conclusion that he must make changes for Wednesday's semi-final, second-leg with Schalke.
"If you look at our experience of last year, the team were very tired against Chelsea in that next game after losing in Munich," he said. "All four semi-finalists lost this weekend too and there are certain statistics that tell you the European games do take it out of you.
"That has to have a bearing on my team selection.
"We have two massive games. Both as important as the other. Chelsea is massive, so I have to pick the right team to ensure I have the same kind of freshness I need for Sunday's match.
"Hopefully I will have the nucleus of a team that is still good enough to win the match."
Source: PA
Source: PA