Eberechi Eze struck twice for the FA Cup finalists either side of half-time to bring Spurs back down to earth with a 2-0 home reverse on Sunday after Thursday's euphoric scenes in Norway.
Tottenham set up a European final with Manchester United after they sealed a 5-1 aggregate victory over Bodo/Glimt, but a side which contained eight changes slipped to a 20th league loss of the season.
It marks Tottenham's worst-ever tally of defeats in the Premier League after they twice lost 19 fixtures in the 1993-94 and 2003-04 campaigns.
To add insult to injury, Kulusevski had to be replaced after 19 minutes, but Postecoglou did at least provide a positive update on the Sweden attacker after he lost Lucas Bergvall and James Maddison to season-ending injuries last week.
"Medical team are not too concerned with him, more of a knock than anything else, so we're hoping he should be OK," Postecoglou said of Kulusevski.
"It's not helpful when you lose another player, it's the fine line we're trying to navigate at the moment. That aside, it was a disappointing performance and nowhere near the level it should be.
"Guys had an opportunity today to put their name forward and put some pressure on the guys who played the other night. Fair to say there wasn't any compelling evidence of that.
"That's what I'm trying to push with these guys, that sometimes in football you've just got to take the opportunities there before you. They're never going to be perfect.
"We made eight changes but there is an opportunity there and you've just got to take it when presented to you and I'm disappointed more didn't step forward today."
Postecoglou decided against bringing on Brennan Johnson, Richarlison and Dominic Solanke due to the physical load they got through on an artificial surface on Thursday night and also kept Cristian Romero and Micky van de Ven on the bench, despite the prospect of a record-breaking 20th league loss.
He added: "I get it and people are banging the drum about this record with defeats, but if people don't see there is a correlation between the two...
"It's not exactly Pythagoras theorem. It's quite simple to understand we would have had much better results if we didn't have to navigate this. So, either you understand or you don't and beat us over the head, I get it."
