Manchester United manager Louis van Gaal has said he believes the intensity of the Premier League has led to English clubs struggling in the Champions League.
If Premier League clubs do not start to perform in the next six to 18 months there is a strong prospect that England could drop to three Champions League places.
Only one club, Chelsea, has managed to reach the last four in the previous three seasons, while none made it past the round of 16 in last season's competition.
And Van Gaal said he believed the draining nature of competing in the top division was the reason.
"The Premier League is a very difficult league," he said. "It is not easy to beat the opposition every week.
"It is always a struggle and a battle in England -- and then you play in the Champions League in midweek.
"That's the difference between all clubs in Europe and Premier League clubs. The difficulty is all these players are in a rat race. It takes a lot out of the players every week.
"That's why you have to rotate [your squad]. That is what I do now, because I have to protect my players."
Van Gaal, who has also coached in Spain, Germany and the Netherlands, said other leagues helped clubs in Europe by bringing domestic matches forward to Friday nights in order to give them more preparation time.
"We have to play Sunday-Tuesday or Sunday-Wednesday," he said. "Between these matches, we cannot recover all the time. That is a problem we have."
However, this season no Champions League team has been scheduled to play on a Sunday during the group stages.
United face Wolfsburg at Old Trafford on Wednesday, looking to bounce back from a 2-1 defeat at PSV in their opening group game.
His fears over the future of English clubs in the Champions League come after former Arsenal star Thierry Henry, now a TV pundit, said there was "a real danger that the Premier League will soon lose one of its Champions League places."