Arsenal manager Arsene Wenger believes the Premier League's intensity hurts English clubs' chances in Europe.
Wenger's side host Olympiacos in the Champions League on Tuesday looking to get their Group F campaign going after a shock loss to Dinamo Zagreb in their opener.
"These teams dominate their leagues," Wenger said.
"Olympiacos won comfortably on Saturday, they were safe after 20 minutes, when we were in a high-intensity game at Leicester.
"Mentally you cannot ease off before a Champions League game, as some others can.
"For example Dinamo Zagreb played on Saturday and they took three players off at half-time. I cannot do that at Leicester.
"For sure the Premier League is demanding and consuming energy wise – mentally more than physically."
"I see him train every day, he is dedicated, focused, hungry," he said.
"His dad said he lacks a bit of self-belief and I can confirm that because he is very critical of himself, but that is positive and a sign of top-level sportsmen.
"You have to, of course, find the right balance, [being] not happy with what you do without harming the confidence.
"He is maybe not happy enough with his performance after the games and sees only what he has not done well, not enough what he has done well."
