Qatar, Saudi Arabia, Kuwait and Bahrain have all crashed to defeat in a tournament being played barely a month after the Gulf Cup of Nations.
Bruno Metsu, who has worked in the region since 2002, said it was tough playing two big tournaments in such a short space of time.
"We see that the Gulf teams all lost their first matches, and we are talking about technique and physical qualities. But what we haven’t talked about is mental stamina," he said.
"It’s difficult to play two big tournaments in two months and I think that you have to ask, are the players capable of withstanding the mental pressure of playing in two competitions back to back?
"It's a mental thing, not about morale. It’s difficult for young players to come back and play in two big tournaments."
Qatar lost 2-0 to Uzbekistan in their opener while Saudi Arabia was stunned 2-1 by Syria, a defeat that cost coach Jose Peseiro his job.
Bahrain went down 2-1 to Asian powerhouse Japan while Kuwait, the Gulf Cup champions, were 2-0 losers against China.
Metsu said the Asian Cup and Gulf Cup were not comparable, with differemt types of opponents.
"You can’t compare the Asian Cup and the Gulf Cup. They are two completely different championships," he said ahead of Qatar's must-win Group A clash with China on Wednesday.
"But when you compare the Gulf teams to the non-Gulf teams, you see that Jordan and Syria have played to their potential and played good matches.
"They played with more aggression and were more fresh mentally. They were much fresher than the Gulf teams such as Saudi Arabia, Bahrain and Qatar."
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