Jose Mourinho believes Real Madrid will struggle to remain competitive if they reach the Champions League knockout stages after being pitted against three European champions.
The Merengues coach could have done without such a daunting set of group-stage opponents as he aims to win his third Champions League crown.
Barclays Premier League champions Manchester City are up first at the Bernabeu on September 18, with the winners of the Bundesliga and Eredivisie, Borussia Dortmund and Ajax, also providing stern tests to Madrid's hopes.
Mourinho insists his players cannot afford to make any mistakes across six gruelling games if they are to stand any chance of reaching the last 16.
He told AS: "There are not only four league champions, but three of the champions from the strongest leagues in Europe - La Liga, Premier League and Bundesliga.
"It seems to me like Italian football is struggling a bit, because only Juventus could compete with the teams we have in our group.
"French football is certainly improving, but it's not at our level yet.
"It's a very tough group.
"And closing it out are Ajax, who are constantly having to rebuild their side but always seem to find new and interesting players, men who want to make a name for themselves and fight for the cause."
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 added.
"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."