The former Everton midfielder had just scored in the KNVB Cup to put Ajax 2-1 up, when an object struck him in the back of the head.
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Klaassen put his hand on where he'd been hit, and upon withdrawing it realised he was bleeding heavily.
The match was then suspended in the 62nd minute with the teams and football association deciding what steps to take.
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During the pause, Feyenoord assistant John de Wolf took a microphone and berated the home crowd.
"Use your brain, damn it," he said. "Make sure nothing else gets on the field.
"Get us back behind the team. We desperately need you. But use your mind!"
After a lengthy delay, play was then resumed, with a repaired Klaassen stepping back out for Ajax.
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However, just minutes into the restart, the midfield sunk to the ground and was substituted in pain.
Despite the sad ending to his night, Klaassen's goal proved to be the matchwinner, booking Ajax a final spot against PSV on April 30.
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