Mourinho's La Liga giants face the intimidating prospect of making a place for themselves with Manchester City, Borussia Dortmund and Ajax set to secure a place in the knockout stages.
"It will not be easy. But I know my players well and I have to admit that preparing for such intense games is often easier than preparing for others," The Sun quoted Mourinho, as saying.
"The players know they have to be at their best and give everything in the Champions League . They need to be even better than they have already been this season," he added.
Mourinho fears his players may not be as competitive in the latter rounds if they come through six gruelling group games.
"There are not only four league champions in our group, but three of the champions from the strongest leagues in Europe - La Liga, Premier League and Bundesliga. We will have to battle minute by minute throughout the group stage, with maximum intensity, concentration and pressure," he said.
"And we will go through to the second round with that effort having taken its toll, both physically and mentally," he added. (ANI )