Goalkeeper Jeroen Zoet believes PSV must "learn a big lesson" after a shocking first-half display resulted in a 3-2 Champions League defeat away to CSKA Moscow.
The Dutch champions, who got the better of Manchester United in their opening Group B fixture, conceded three goals before the break to leave themselves with a mountain to climb in Russia.
The visitors threatened a comeback with a brace of goals after the interval from Maxime Lestienne, but they finished the game with only 10 men following the dismissal of Santiago Arias.
Zoet – who committed the foul that allowed Seydou Doumbia to make it 3-0 from the penalty spot – admitted PSV paid the price for what he described as "small mistakes" at the back.
"It is a crying shame," he said. "It was deadly quiet in the dressing room after the match. We all realise that we gave it away here. That is just not allowed to happen.
"There was just too much unrest in our defending and we tried to find a footballing solution for everything.
"Small mistakes are just ruthlessly being punished at this level. We have to learn a big lesson out of this. This is really a shame as we really could have gained some points here."
PSV's next European outing sees them travel to Wolfsburg - all four teams in the group have three points to their name.
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