Nemanja Matic said Chelsea is in no position to expect a win against a Manchester United side that has lost four games in a row.
Both Chelsea and United have struggled this season, with the former languishing in 15th position on the Premier League table, while United have endured four losses in a row for the first time since 1961, but remain in sixth place, three points off fourth spot.
Chelsea could only manage a 2-2 draw against high-flying Watford at Stamford Bridge on Boxing Day.
Chelsea travel to Old Trafford under the guidance of interim manager Guus Hiddink, who replaced Jose Mourinho after he was sacked, with United manager Louis van Gaal under increasing pressure.
"We are in worse position than Man United. They are sixth and we are 15th, and we cannot say that mentally we are better than them," Matic said.
"Of course, when you lose four games in a row it is not easy, and maybe we will try to exploit that.
"But we have to know that they also want to win and will give everything to win. They also think they don't deserve to be in sixth position so we have to be ready for everything."
Another loss could seal Van Gaal's fate with former Chelsea manager Mourinho heavily linked to the job.