Spurs lost Maddison and Van de Ven to injuries before halftime in Monday's 4-1 home Premier League defeat by Chelsea. Maddison withdrew from England's squad earlier on Friday for their last two Euro 2024 qualifiers.
The injuries mark a bittersweet week for Postecoglou as Spurs lost top spot but the Australian won a third straight Premier League Manager of the Month award. Tottenham are second in the standings, one point behind champions Manchester City.
"You don't get a lot of time to enjoy the moment, do you? It's fair to say there was a fair bit that happened after Monday night," Postecoglou told reporters ahead of Saturday's trip to Wolverhampton Wanderers.
"Micky with a hamstring injury we knew was fairly significant. We're looking at a couple of months, looking into the New Year.
"Madders (Maddison is) a lot worse than we first thought. It was bad the next day. Again, into the New Year with him."
Forward Richarlison is also sidelined after he underwent groin surgery which will keep him out of action for a few weeks.
"Richarlison had the operation which we kind of scheduled anyway, he's been battling with that since day one and the international break is coming up. He should be a month if everything goes well," Postecoglou added.
"We've got a couple of suspensions (Cristian Romero and Destiny Udogie) as well ... But Ben Davies is back and available. Pedro Porro is fine and available."
Injuries and suspensions mean Postecoglou is without three of his regular back four.
"If it was just Micky, you'd miss him because of the qualities he has. It wouldn't disrupt us as much," he said.
"But we've got to bring in three different players in a back four, that's the big challenge for us."
Postecoglou said the number of absentees gives second-string players such as Giovani Lo Celso, Bryan Gil, Oliver Skipp and Eric Dier a chance to impress while they would be careful with Rodrigo Bentancur following his return from a knee injury.
"That is why they have been training hard. They have been training with us, they haven't been separate from the group," he said.
"Fair to say the opportunity comes now for a few of them. It's up to them now to take that opportunity, but they've been preparing themselves for that since the first day."
