McClaren does not believe Schalke have the firepower to damage United's defence.
"I would rather have United's offensive players than those of Schalke," the former Wolfsburg coach and United No2 told the Daily Express. "But there is absolutely no disputing the fact that in Raul they have one of the best goal scorers in the world and a player who is always dangerous.
"He is defying his age and has adapted brilliantly to the way Schalke play in Europe.
"Schalke are a strange team. They lost in the league at the weekend and their Bundesliga record is pretty indifferent. But put them in the cups and they are clearly a different side. That means they are very dangerous and United will be aware of how they can raise their game for big matches, and they don't come an awful lot bigger than Champions League semi-finals."