De Ligt, who scored a late equaliser against Tottenham Hotspur in the Premier League on Saturday, returns to the squad in place of Inter Milan's Stefan de Vrij, who has been dropped for World Cup qualifiers against Poland and Lithuania.
The Dutch have a three-point lead over Poland and a much better goal difference and will qualify if they win in Warsaw on Friday. If not, they are expected to secure their berth at next year's finals when they host Lithuania in Amsterdam on Monday.
"Partly, it is based on playing time, although Nathan Ake isn't playing 50% of the (Manchester City) matches, but he's part of this squad," Koeman explained about the 26-year-old De Ligt at a press conference on Monday.
"You decide separately for each player in the squad. I felt I should select Matthijs based on what he's showing.
"He's been through a difficult period. I've spoken to him and told him he didn't look confident in his actions, but now that's much better. And his position at the club and the club's performance will always be important," the coach added.
As for Ake, Koeman said the fact he was left-footed helped balance the number of right-footed defenders in the squad.
"Nathan himself is also concerned about the fact that he doesn't play much. He is one of my captains, though," Koeman told reporters.
"He's a left-footed player, while we generally have a lot of right-footed central defenders. That makes all the difference having him in the squad. He's an important player who has always been there when I've needed him."
