De Ligt, 20, was criticised for his performance in Juve's thrilling 4-3 Serie A victory over Napoli at Allianz Stadium.
The Netherlands international, signed for €75million from Ajax in July, came in to play alongside Leonardo Bonucci due to Giorgio Chiellini's knee injury.
Netherlands head coach Koeman said the centre-back's display was below his best.
"What is a shocker? It was also a question why he didn't play the first match. Then he played because somebody else had an injury," he said. "He didn't play well. He had a bad moment with the goal of [Hirving] Lozano. The other two goals were different. It was a free-kick that turned in and that's always hard to defend.
"But yes, he didn't play the way we want to see. I spoke to Matthijs today after lunch."
Koeman suggested De Ligt was struggling to adapt after his move from Ajax and also said he may be getting used in the wrong position.
"Sometime people take change too easily. I still have the newspapers of my first match at Barcelona. It was a disaster. We lost 2-0 and everybody was questioning my arrival," he said.
"But if you asked them now about me I bet it's different. So Matthijs is 20 years old. On the television they take changing to a new club very lightly.
"But I wouldn't position him left in defence. I would have put Bonucci there. He is more experienced."