Down 10-0 on Friday night, the introduction of Smith at hooker and Waerea-Hargreaves changed the game when they were brought on by Roosters coach Trent Robinson midway through the first half.
The duo exploited the Rabbitohs' tiring middle, with Smith scoring off the back of a Waerea-Hargreaves play-the-ball which helped turn the tide and set the Roosters on their way to a 20-18 victory.
While Friday's derby wasn't as spiteful as last year's finals game which led to a record seven sin-bins, Waerea-Hargreaves was able to keep his aggressive streak in check.
The 34-year-old has a tendency to overstep the mark but he refused to be baited into any conflict as he helped the Roosters win the yardage battle.
"He said 'I'm going through these guys' and everyone jumped on the back of that," Smith said.
"Without Jared today we wouldn't have won the game.
"I've had the privilege of rooming with Jazza every (Kiwis) tour we've been on, so I know what he's about and I know what he brings.
"But this game here was something new, his mentality towards controlled aggression, the way he spoke to the team out on the field when the scuffles were going on and how he stayed composed - I was quite proud of him.
"I know that's something he's been working on.
"We don't want Jared playing one week on and three weeks off, we want him all the way through the season and I thought he took a big step towards that controlled aggression without being soft."
The meeting with Souths was Smith's first taste of the derby since joining the Roosters and he likened the atmosphere at Allianz Stadium to playing in a grand final.
"I've come into the club and everyone is just a little bit more focused than usual, a little bit more antsy than usual," he said.
"Robbo yelled at me at the start of the week, so it must be a Rabbitohs game."
Smith played through a heavily bruised oblique and will welcome a bye before facing Parramatta in round five.
"If it wasn't the Bunnies I probably wouldn't have played, this was a game I wanted to be a part of," he said.
"I took a record amount of anti-inflammatories to keep me out there, thank the Lord for Naproxen."
