The Hammers host United today in what is a key game for both teams as they fight for points at opposite ends of the table.
“Manchester United play Chelsea in the Champions League next week, so they have two very tough games in London within the next five days,” Sullivan told The Telegraph.
“They are far less daunting on our ground than at Old Trafford, where they are almost unbeatable.
"Our players are very fired up for Saturday and I’m sure after 90 minutes the United players will know they have been in a match.”
West Ham currently sit just one place above the relegation places on goal difference.
“We are playing for three vital points,” Sullivan said.
“Every game is like a cup final for us now - in fact more important than a cup final. The matches are incredibly stressful to watch, after every game I feel mentally and physically shattered. We are paying the price of only winning two of our first 18 games. It seems like every game is life or death.”
West Ham manager Avram Grant has added to Sullivan’s comments, claiming that despite believing United are not the same force away from home, their success is more down to the management of Sir Alex Ferguson.
“The strength of Sir Alex is that he can keep the team [going] even when they are not so good,” Grant said.
“They started the season not so well and still took points.
"They started the season not playing good football, now they are, but you never heard any stories. They believed in themselves even when they were not making four or five passes like they used to. This is why they are in first place.”