The 19-year-old moved to Turin from Ajax for £68 million ($84m) back in July, bringing to an end his 11-year association with the Dutch giants.
De Ligt earned the big-money transfer after a superb 2018-19 campaign which saw Erik ten Hag's side win a domestic double, while also reaching the semi-finals of the Champions League.
After months of speculation, De Ligt decided Juventus was the best place for him to continue his development, but he has yet to make his competitive debut for the club.
The ex-Ajax star was an unused substitute during Juve's 1-0 win at Parma on Saturday, which even he admits came as a surprise decision from head coach Maurizio Sarri.
Leonardo Bonucci and Giorgio Chiellini played the full 90 minutes, with the latter grabbing the winning goal in the 21st minute of the game.
Koeman, who guided the Netherlands to the UEFA Nations League final earlier in the year, has hinted that he may have to make a tough selection decision in the near future if De Ligt isn't playing regularly at club level.
"On the one hand, you assume that he is going to play because he makes a transfer for a lot of money," Koeman told Studio Voetbal: "But if a club has Chiellini and Bonucci, it may be that the coach makes a different choice.
"The moment he chooses Juventus, he goes to another country and he has to learn a new language. That takes time, but of course, that should not take too long. I am not worried yet, but if this is still the case in October, then you will start thinking about the fact that he has no rhythm."
The Netherlands boss also insisted that De Ligt is still adapting to a new style of play in Italy, adding: "I spoke to Matthijs two weeks ago, then he also indicated that he had to adapt to the way of playing and defending.
"It is something different than what he was used to at Ajax. In Italian football, certainly in his position, it's going to be about zero (keeping a clean sheet). It's less about the structure, like ours in the Netherlands."
De Ligt will be hoping he gets plenty of minutes under his belt next Saturday before heading away on international duty, with Napoli set to arrive at the Allianz Stadium.