Real Madrid coach Jose Mourinho has big concerns over a tough Champions League group.

Mourinho is also concerned that the heightened level of competition in the early stages will have a debilitating effect on Madrid's chances of reaching the Wembley final.

"The difference between our group and the others is that we cannot afford to make any mistakes. Neither can our rivals," he said.

"Something else to consider is the two teams who qualify from this group won't go into the knockout stages in the same condition as the other 14 who qualify.

"We will have to battle minute by minute throughout the group stage, with maximum intensity, concentration and pressure. And we will go through to the second round with that effort having taken its toll, both physically and mentally.

"The other clubs will be fresher, without cards or suspensions and able to rotate their squads.

"We will get to the quarter-finals through group effort, probably having picked up a lot of cards over a lot of minutes, and being both physically and mental worn out.

"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.

"I am reassured, though, that Real Madrid is considered such a big scalp.

"(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.

"We need to start winning from the very first minute of the match against Manchester City at the Bernabeu."