Just when Man United hoped to be wrapping up a record 19th English league title, the team's lead was cut to three points after losing at Arsenal on Sunday.
Ferguson's side will be toppled from the Premier League summit on Sunday with a loss to Chelsea, which has the week off after being eliminated by United in the Champions League quarterfinals.
Ferguson blames Sunday's loss at Arsenal on the lack of recovery time after winning 2-0 at Schalke in the first leg last Tuesday.
"All four semifinalists lost this weekend too and there are certain statistics that tell you the European games do take it out of you," Ferguson said on Tuesday. "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 tomorrow 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."
Wayne Rooney, who scored one of the goals in Germany, is set to miss the second leg with a hamstring injury, while Javier Hernandez is likely to be rested as Dimitar Berbatov and Michael Owen prepare to start up front.
"Let's not forget that Dimitar is still our top scorer," Owen said. "He has only not been scoring lately because he has not been playing as much. Overall, the strikers have dovetailed quite well. Everyone has chipped in at certain parts of the season.
"Wayne almost carried us right the way through last season. This year other people have come in to fill the void. It has been a great effort all round."
Ferguson maintains that he is not underestimating Schalke, which has lost its last three games and is out of contention for the Bundesliga title.
"It is always dangerous in football to assume you have cracked it; that you have found a silver bullet or discovered some secret formula for success," Ferguson said. "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.
"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 tomorrow and it certainly will not be."
Schalke's plans were disrupted by a two-hour delay on Tuesday on the way to Manchester due to visa issues for its players from outside the European Union: Jefferson Farfan (Peru), Atsuto Uchida (Japan), Ali Karimi (Iran), Junmin Hao (China) and Edu (Brazil).
Barcelona or Real Madrid, playing today, await the winner in the May 28 final at Wembley Stadium.
"The main things is to believe in ourselves," Schalke coach Ralf Rangnick said. "Why should we capitulate? Stranger things have happened in football."
