Spurs go to Leicester, with the Foxes sitting inside the dropzone.
But Postecoglou said this morning: "It'll be a tough game.
"They're in a fight to stay in the Premier League and they'll come and try and make it difficult for us."
Spurs go into the clash having won impressively at Europa League opponents Hoffenheim last night.
Postecoglou said: "Outstanding effort by the players in a really important game for us in European context. Positive result help us with our target. I thought the lads gave everything on the night, really pleased with the outcome.
"I thought Archie (Gray) out of position was great, Lucas (Bergvall) worked his socks off for the time and Mikey (Moore) got an assist.
"There's no doubt we will get benefit from these players for many years to come, they've had a tough start to their careers. I think it's hugely positive."
On Callum Olusesi's debut, he continued: "Callum has been great, he's a very talented young player and one we've high hopes for. He's still got a long way to go to becoming part of the first-team but hope the experiences they gain fuel them even further."
Meanwhile, Postecoglou admits Cristian Romero isn't yet ready to return to action.
"Like I said, we got him back training.
"Obviously we haven't been training a lot and because the other day was sort of low tempo session, it was just good to get him involved, not just for him but the players involved because they need a bit of help and encouragement and Cuti training with them gives them a big lift."
He also said: "We should have light at the end of the tunnel in a couple of weeks, it's not as dire but at the same time there's always a possibility that before those guys come back we might lose a couple of these guys (current players in form).
"That's always the challenging bit, that's what we're going through.
"It's been two months of hard grind for the players, just trying to support them as much as we can. Our schedule is not going to ease up at all.
"The resilience and the determination of the players to get out there and perform is a fantastic thing for me to witness, couldn't be more proud of them."