The Tottenham squad watched United's 5-0 destruction of Bayer Leverkusen on Wednesday.
When asked if the champions still have the same cache without Ferguson, Sigurdsson said: "I think so.
"They never start the League too strong but during the business part of the season they are always very strong. They have been there and done it before, they have some world-class players and are the team everyone wants to beat. I saw some of their game against Leverkusen. It was 2-0 when I stopped watching it. Some players they have are world-class and we know we need to stop them, like Rooney among others.
"We just have to focus on ourselves and play attacking football."
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