"This is more than three points. Victory on Sunday will have double value," Deco said in the Daily Star.
"It will provide a huge gap between us and United. It will give United a message that we are ready to challenge.
"I am only too aware of the defeat in Moscow and what it meant to my team-mates.
"I have been waiting for this game since I arrived at Chelsea and against United it will be a test of strength."