Son has only scored four goals for his club this season and just one from open-play in his last 12 appearances for Spurs.
The South Korea international struggled with a hamstring issue earlier in the campaign and has cut a frustrated figure at times since his return, but manager Postecoglou backed the 32-year-old to return to his best ahead of Thursday's Premier League trip to Bournemouth.
"I think there were questions about Mo (Salah) maybe a year or so ago," Postecoglou pointed out.
"Go for it! It might help me if you write him off. I don't get that sense. I still feel from a physical output and the quality he has and certainly from a mentality point of view, I haven't seen it.
"What I do know is, from my own experience, the end comes pretty quickly. It's not falling off a cliff, but you find with a lot of great players that all of a sudden it's gone from bang to bang. It's not a slow deterioration.
"I don't see that in Sonny, I still think he's got a hell of a lot of quality and from a physical standpoint, the way he looks after himself, I don't see his abilities diminishing just due to the age factor at the moment. I don't see that.
"He's had a bit of a disrupted start this year where's he's had a couple of setbacks where he hasn't played regularly. I think with Sonny, the more regularly he plays the increase in that output.
"So, the answer is no, I don't think so and I'd be wary about writing him off."
Son had to be used through the middle in Sunday's 1-1 draw at home to Fulham after Dominic Solanke was sidelined on the morning of the match with illness.
Solanke was set to return to training on Wednesday and will hope to be fit enough to make a first return to Bournemouth since his summer departure after 77 goals in 216 outings for the south coast club.
While Solanke should return at Vitality Stadium, Postecoglou will be without Guglielmo Vicario (ankle), Micky van de Ven (hamstring), Cristian Romero (toe), Rodrigo Bentancur (suspended), Wilson Odobert (hamstring), Mikey Moore (virus) and Richarlison (hamstring).
Archie Gray appears in contention to earn a first Premier League start, potentially in place of Pedro Porro at right-back and Dejan Kulusevski will return after being rested versus Fulham.
However, Postecoglou acknowledged unless Tottenham's list of absentees improve, they may have to alter plans for the winter transfer window.
"January is a tricky one. I wouldn't expect us to do too many significant things in January but if January 1st, we're in exactly the same position we are now with nine players out, I reckon we'd be doing something," Postecoglou confirmed.
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