De Ligt partnered Leonardo Bonucci in the centre of the Juventus defence in the absence of Giorgio Chiellini, who is set for a long spell on the sidelines with a knee injury, and has widely been blamed for all three of the visitors’ goals.
The 19-year-old, who also scored an own goal against Inter Milan in pre-season, was making his first league start of the campaign after sitting out Juventus’ opening game against Parma.
Koeman believes De Ligt was played out of position in the game as that contributed to his dismal outing.
"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," Koeman told a news conference on Monday.
"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 expects the former Ajax captain to improve with time, and likened De Ligt’s situation to his own early days at Barcelona, where he won four league titles and the UEFA Champions League in the early 1990’s.
"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."
