"For coaches to prepare a team with one week is very, very difficult, obviously," said Klinsmann, who was twice a World Cup coach, with Germany in 2006 and the United States in 2014.
"There have been extreme mental challenges for the teams," he said, adding that the expanded squads with 26 players instead of the usual 23 left coaches "dealing with 15 unhappy players."
Two tactical trends were highlighted by Wenger from the 48 games in the group stage - the importance of attacking from wide positions on the field and goalkeepers' ability to pass the ball as an outlet for teammates being pressed by opponents.
"I personally think the team with the best wide players will win this tournament," Wenger said.
He said attacking teams were forced to find space on the flanks because opponents now better protect the center of the field, and this also led to fewer long-distance shots being taken.
Evolution of the "sweeper keeper" role pioneered by Germany captain Manuel Neuer saw a 70% increase since the 2018 World Cup of goalkeepers offering to receive the ball with their feet.
"It is a fascinating development of the position of the goalkeeper," Klinsmann said, adding youth training likely needs to adapt so potential keepers also spend time playing an outfield position to develop their ball control and passing skills.
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