Sir Alex Ferguson is refusing to underestimate Schalke ahead of their UEFA Champions League semi-final showdown on Tuesday.
United are heavy favourites to overcome Schalke and book their place in next month's final at Wembley.
However, Ferguson is wary of the threat posed by the Germans after they dumped out holders Inter Milan in the quarter-finals.
"They [Schalke] have beaten every team they have played in the Champions League at home this season, including Valencia and Inter Milan," Ferguson said.
"Taken in isolation, that's fantastic form and there is no way Manchester United are going to be led into believing that this is easy. This is a difficult game for us and we have no choice but to treat it that way."
Ferguson watched Schalke's second-leg victory over Inter at Veltins Arena and was left impressed by the Bundesliga outfit.
"They never looked like losing the game and that's quite a credit to them in terms of playing a side like Inter Milan," added Ferguson.
"They always looked in complete control and, even in the good spells that Inter had, they [Schalke] never looked like losing. It's a semi-final now and, any semi-final you go into, you know it is going to be difficult."
Ferguson is also refusing to get too ahead of himself and contemplate a final date with either Barcelona or Real Madrid.
"I don't like jumping ahead," continued Ferguson. "We have to deal with Schalke first. If we can manage that, I don't care who we play in the final."
Who will contest the UEFA Champions League final?
Schalke and Barca
Schalke and Madrid
Man Utd and Barca
Man Utd and Madrid