Tottenham's injury woes continue with Romero hamstring strain
The 25-year-old Argentina centre-back's absence is all the more galling for Spurs as Romero only returned from a three-match suspension at the beginning of December.
Romero suffered the injury during last weekend's 2-1 victory against Everton with Postecoglou, speaking ahead of Thursday's match at Brighton, saying: "Not so great with Romero.
"He had a scan the other day and has got a hamstring strain. We are looking at probably four or five weeks for him.
"Disappointing to lose him, for sure. We obviously missed him with the suspension and now we've just got him back and he steadied things up.
"Now he will be missing again for quite a chunk of time."
Romero's fellow central defender Micky van de Ven remains sidelined with a hamstring injury of his own sustained during a 4-1 loss to Chelsea in November.
Van de Ven suggested last week he could be back training in a fortnight, with a cautious Postecoglou hinting the January 14 trip to Manchester United as a comeback date for the player.
"He is getting closer," Postecoglou said. "We will see him some time in January. I don't think Burnley but Man United on the 14th potentially
"I don't look at comeback dates until I see them training with us and he hasn't been part of the group.
"He is not due to be part of the group this week so we'll see how he is going at the end of this week. Once they start training with the group, that's when we start thinking about if they are available.
"He is at the tail end of it, for sure and at some point, middle of January he could be available but how we use him after such a long absence will be depending on where he is at."
Spurs could leapfrog Aston Villa into third place in the Premier League with victory at Brighton.