The Tottenham captain has missed the club's last two matches against Wolverhampton Wanderers and Eintracht Frankfurt due to a foot problem.
In Son's absence, Spurs fell to a 4-2 defeat in their Premier League meeting with Wolves, before they recorded a memorable 1-0 victory in Thursday's Europa League quarter-final second leg against Frankfurt, setting up a semi-final tie with Norwegian side Bodo/Glimt.
Tottenham are now preparing for Monday's tricky clash against a Forest side that are sitting in third place in the Premier League table.
Unfortunately for the hosts, Postecoglou has confirmed that Son will not recover in time to feature against Nuno Espirito Santo's side.
Postecoglou confirms Son absence
"Sonny will still be out," Postecoglou told reporters in Saturday's pre-match press conference. "He's been dealing with this foot problem for a while, so now is as good a time as any to give him the right time to get over it, so he'll definitely miss Monday night as well.
The Tottenham boss was unable to provide any clarification when asked about a potential return date for the South Korea international.
"Not really sure, something he's been dealing with for a little while, and we got to a point not only wasn't it improving, but it was deteriorating," Postecoglou said.
"Rest is going to be as important as anything else with that kind of injury, so we'll give him that time.
"Sonny's the kind of guy who always wants to train, so now we're going to have to put him on ice for the next few days and see how he responds to that."
Postecoglou delivers Maddison update
Tottenham also have concerns over Maddison's physical status after he was forced off in first-half stoppage time against Frankfurt.
Frankfurt goalkeeper Kaua Santos gave away a penalty when he clattered into Maddison in an incident which led to the award of Dominic Solanke's match-winning penalty.
Postecoglou has admitted that it is currently unclear whether the 28-year-old will be available for Monday's home fixture.
"We had the day off yesterday as we didn't get back until the early hours of Friday morning," Postecoglou said. "But fair to say he'll [Maddison] probably be a bit sore, so he's probably a doubt for Monday.
"The rest of the group worked hard, so they're a bit sore but should be okay for Monday night."
Tottenham remain without the services of defender Radu Dragusin, who is out for an extended period after he sustained an anterior cruciate ligament injury in February.
Written by
Ben Sully