Dutch international centre-back Van de Ven suffered a hamstring injury while midfielder Maddison injured his ankle in Monday's 4-1 loss at home to Chelsea.
Spurs also had Cristian Romero and Destiny Udogie sent off during their first defeat of the league season, which means Postecoglou will be without several key players for Saturday's trip to Wolves.
"Fair to say a fair bit happened after Monday," Postecoglou said on Friday.
"Micky obviously with that hamstring injury, we knew it was fairly significant, probably a couple of months for him looking into the new year.
"Madders is a lot worse than we thought. He came off with an ankle injury and the next day wasn't great, so we sent him for a scan. Again probably into the new year for him."
Tottenham, who are second in the Premier League table, a point behind champions Manchester City, earlier Friday confirmed that Maddison had withdrawn from the England squad for this month's Euro 2024 qualifiers.
