Rangers have handed indefinite bans to travelling supporters who were filmed making racist chants and gestures about the Celtic striker at the weekend.
Furuhashi has acknowledged the support of his own club and supporters, writing on social media: "I'll never walk alone."
Former Yokohama Marinos manager Postecoglou vowed that he would do everything possible to protect the 26-year-old after making him one of his priority signings after arriving from Japan himself in the summer.
The Greek-born Australian told Celtic TV: "To be honest, it saddens me a lot because I'm the one who's brought him over here.
"I spent three-and-a-half years in Japan, and I was a foreigner, and I was welcomed with open arms over there. I know how polite and respectful the Japanese people were, and I'm the one that brought Kyogo here.
"I'm sure his experience is totally the extreme opposite of what's been portrayed in the last 48 hours. Talking to him, he loves being here, he loves the city, he loves the way everyone's embraced him.
"It's not about education, people are more than well aware of what's right and wrong - just be a decent human being and treat people with respect.
"Even for myself, I've been lucky enough to walk around Glasgow with my family and you see all the different nationalities and different establishments - whether that's restaurants, businesses - and it's obvious that people have come from all over the world to make this city a great city.
"I love that - that's kind of my background. I'm an immigrant. And to hear that, even if it is a minority, it shouldn't be there and it does sadden me.
"Our number one priority is to protect and support the player. He's a cracking guy. If anyone ever comes across him, he's such a nice young man and he doesn't deserve any of that type of behaviour. No one does, to be honest, no one does."
Celtic fans have quickly taken to Furuhashi after he netted six goals in his opening six starts and Japanese flags can be seen fluttering around the stands at Parkhead.
"That's the beauty of football," said Postecoglou. "When you walk around Celtic Park you'll see images, whether that's (Shunsuke) Nakamura or Henrik Larsson, or whoever it is.
"When you start bringing people from outside your own space, whether that's your nation or even your neighbourhood, that's when it enriches it. It enriches people's lives.
"That's why people leave their homes and come halfway across the world, because they're good at something, but it actually enriches the place that you go to.
"I've always loved that about football. I always thought it was a unifier because, in terms of football, it didn't matter where you were from, what economic status you might have had or whatever your background was, you could play this game, and it brought people together.
"And having Kyogo here, or Liel Abada here - young men who I think put smiles on people's faces - I don't see how people try and make a negative out of that."