Dest has been in and out of the Barca team with a thigh problem since the turn of the year but was picked ahead of Oscar Mingueza to deal with the threat posed by PSG forward Kylian Mbappe on the left.
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However, Dest struggled to get to grips with Mbappe, who scored a hat-trick, before the ex-Ajax right-back was replaced by Mingueza in the second half.
"Maybe because he has not been playing regularly and because he has had some injuries, that was an influence on his participation in the team," Koeman said in a news conference on Saturday.
"But now he is OK, he's physically OK. He is  years old but he needs to learn, he needs to be more aggressive, he needs to be more concentrated. He had too many periods in the game where he's not 100% focused. We spoke to him about this."
Dest joined Barca from Ajax for an initial fee of €20 million in October and has made 24 appearances in all competitions this season.
The USMNT right-back got off to a good start, impressing in the Clasico defeat to Real Madrid at the end of last year, but a thigh injury has disrupted his form in recent months.
Koeman, who pushed hard for the signing of Dest last summer, believes he can still become a key player for the Catalan side, but warned him playing for Barca is completely different from playing for Ajax.
"He's a young player, that's good, he can learn, but he needs to learn," the Dutch coach, who also played for Ajax and Barca, added.
"With all the positives he can bring to the team, he [also] needs to learn it is a big step to play for Barcelona than to play in Holland. He needs time, he needs advice, but it starts with the player himself."
Dest has been included in the Barca squad to face Cadiz at Camp Nou on Sunday in La Liga as they look to bounce back from Tuesday's Champions League round-of-16 first leg loss to PSG.
Barca's season has been written off since then, with comebacks needed to stay in Europe and the Copa del Rey, while they are nine points behind leaders Atletico Madrid in the league.
"The Champions League is very complicated from here but in the Copa, with better finishing and a bit of luck, I think we can turn it around [a 2-0 semifinal deficit against Sevilla]," Koeman said.
"Until the tie has been played, it is not over. I believe we can get through. And we are in good form in the league, even if we are depending on teams above us dropping points. I am not as negative as you suggest."
Barca are still without Sergi Roberto, Ronald Araujo, Philippe Coutinho and Ansu Fati through injury against Cadiz, although Gerard Pique could feature again after making an impressive recovery from knee ligament damage to start against PSG.