The Dutchman told icons.com: "We played Newcastle in pre-season and beat them 3-0 at St James' Park but I do not believe for a moment that it was the real Newcastle on the pitch that day.
"They showed in the way they recovered from losing their first three games in the competition yet still qualified that they have the true English spirit and teams like that are always difficult to beat.
"They have some exciting young players as well as some experienced men like Alan Shearer, which is always a strong combination, and after the way they rescued themselves they will not be short of confidence. We will not make the mistake of underestimating them."
United fly to Spain to face Barcelona on December 10.
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