Speaking this morning, Koeman confirmed Dest was undergoing a medical today ahead of signing permanently for an initial €20m.
He said, "Sergino is undergoing his medical examination and he still has to sign his contract.
"As long as he hasn't signed, I prefer not to talk too much. We will be able to count on a young fullback. It is also about making a difference for the future.
"He will have to compete with the other fullbacks. He is young and played in the Champions League. I'm absolutely sure he will be useful to us."
He continued: "These are the decisions of the club and the player. As we have seen, I will be counting on him.
"He hasn't played from the start because there are more players and Ansu (Fati) has proven to be very good for this team. He was better physically the first few weeks. But today, he trained well and I will count on him."