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."