Overmars was forced out of Ajax after claims of inappropiate messages to female staff.
The Dutchman eventually took charge of Royal Antwerp, but suffered a heart attack and is now recovering.
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"I stand firm that I find it a disgrace the way Marc Overmars has been treated," Ten Hag told VI. "The departure of Marc Overmars killed Ajax at the time.
"And from club perspective it was really stupid to throw him under the bus straight away.
"Overmars was the binding factor within the club. He didn't write a single letter on paper, but every outgoing and incoming line was created by him.
"He was smart, he had strategic insight. When they got rid of him, everything collapsed."