The 19-year-old hasn't signed his contract yet.
The coach hails the promising right-back for his skills and experience.
"Dest's having medical, still has to sign the contract,": Ronald Koeman has just told ESPN during the pre-match press conference.
On the new signing, he added: "He's a young full-back, it's about having young players for the club's future.
"He has to compete with the other full-backs we have. Despite age, he's played a lot of games, played in Champions League. A very useful player."
The 19-year-old landed in Barcelona on Tuesday night.
It's expected that Dest would sign a contract until 2025, with Barca paying Ajax around €25m for the transfer, split in the instalments.
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