Ajax have agreed to sell midfielder Steur to Newcastle United, with the 18-year-old set to sign a five-year contract in the Premier League - a decision that has drawn sharp criticism from Derksen.
"I think it's really dumb of Ajax to have got rid of Steur," Derksen said, opening his critique on Vandaag Inside on Tuesday. "For 28 million euros. But if Ajax tie him down, in five years you'd get more than double that."
Derksen went further, insisting the club had let a generational talent slip away just as he was needed most.
"Because that's a genuine world-class player. And that's exactly what you needed now to bring the glory back to the new Ajax."
Steur had been under contract at Ajax until the summer of 2028, and Derksen was scathing about the club's handling of contract renewal talks.
"He didn't want to sign a new deal. But of course that's what the agent said. Then you can just walk away for free," he said, arguing Ajax should have moved far more aggressively to keep him.
"Then you should have offered him a ridiculous salary. Because he's the biggest talent Ajax have. And you can't make up for that with Daley Blind just because he speaks Spanish," Derksen added, referencing new Ajax head coach Michel, who arrived from Spain.
Derksen argued the club had prioritised short-term financial gain over building a team capable of matching Steur's homegrown quality.
"You could have built a team around a real Ajacied like that. And they went for the money in the short term in a vulgar way, while his value is only going to keep rising."
Fellow pundit Rene van der Gijp shared similar doubts about the wisdom of the transfer, questioning whether a move straight into an unproven Premier League environment was the right step for a player yet to be capped at senior international level.
"It's generally much better for a Dutch national team player to go to Arsenal or Newcastle. You arrive as something," he said. "Those players say to each other: did he play at the World Cup? No. Then he probably isn't that great. Such a shame, I think it's a shame."