Jack Ross’ side have imploded following a terrific 1-0 home win over AZ Alkmaar in the Europa Conference League qualifier early in the month.
The Taysiders collapsed spectacularly in the return game in the Netherlands to lose 7-0 and go into the cinch Premiership game against the champions on a four-game losing streak having conceded 15 goals with one scored.
However, the Hoops boss believes his team must prepare as if they were playing a United side on top form – because they might just be this weekend.
Postecoglou, whose side have won four out of four this season, said: “They are struggling, it is the nature of football.
“If you said a couple of weeks ago after they beat AZ at home that they would be in this situation – but that is the nature of football and that’s how quickly it can change and that’s how quickly it can change again.
“I’m sure they will be looking at our fixture on Sunday, where they want to turn around their fortunes, saying a strong performance against us is a good way to go about it so we have to be ready for that.
“We have to ignore the fact that they are struggling and not think that they are going to come out weakened and make sure we are preparing, as we always do, for them to be at their best, to make it difficult.
“We know it is not going to be an easy fixture and from our perspective it is important we continue on our form.”
Celtic new boy Sead Haksabanovic trained with the squad ahead of the trip to Tannadice but is unlikely to start.
The 23-year-old Montenegro winger signed a five-year contract from Rubin Kazan this week and Postecoglou may take time to bleed him into the team.
He said: “It is more about getting him adjusted to the physical levels we need, we just have to be careful of when we introduce him because the last thing we want is to put him in too early.
“We will see how his fitness levels are over the next few weeks.”