Missing a host of their stars to State of Origin and injury on Saturday, the Rabbitohs refused to die when trailing late in the second half.
They found consecutive tries through 18-year-old winger Tyrone Munro and 21-year-old back-rower Tallis Duncan to give the Bulldogs an almighty scare.
Demetriou said more than half the club's salary cap was missing with Cody Walker, Cam Murray, Keaon Koloamatangi and Damien Cook at Origin and Latrell Mitchell and Campbell Graham out injured, but felt their replacements' grit could prove a huge boost.
"Sometimes, you're looking for moments in your season that define you and I think that was one of those," he said.
"We had every reason not to fight and find excuses and we didn't do that.
"Some of our young players were outstanding, Ty Munro again, shows what he's going to be about, Tallis Duncan was outstanding in the second half in particular.
"Massive effort from Isaiah Tass to play in the back row and (Tevita) Tatola to punch out 60 minutes straight in the middle third."
None caught the eye more than Munro, who again flashed prodigious talent by scoring an impressive double, adding to the try he scored on debut last weekend.
The Rabbitohs are set for a bye and are expected to get Mitchell and Graham back for the round-21 clash with Brisbane.
Demetriou is excited to see the teenage sensation team up with the returning stars.
"I'm looking forward to seeing him play with some of our big players," he said.
"He's come into first grade, he's done his job and he's done it well.
"The beauty about the next couple of weeks is we're going to be healthy and we're going to have a lot of players available.
"When I talk about moments, they're the moments you're looking for … some of your young players that you've put a lot of time and effort in over the last three years.
"You're starting to see that fruit come to life and that'll be important for the energy it brings to our team in the back end of the year."
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