Premiership-winning coach and Nine Network commentator Phil Gould described Napa's first-half performance as: "one of the best defensive displays I've seen in 40 minutes of football from a forward, ever".
The Rabbitohs looked the more likely in the opening exchanges, with Kirisome Auva'a blowing two near-certain tries, before the Roosters struck first with Jackson Hastings chipping perfectly for Daniel Tupou.
When Tricolours skipper Jake Friend sent Aidan Guerra crashing over, it looked like they would head into the sheds with a 10-0 lead but Aaron Gray pulled one back for the home side.
Napa provided the only highlight of an ugly second half when he pounced on a Hastings grubber to give his side a 14-point advantage.
The Roosters looked like a renewed and more-disciplined side, with the win relieving pressure on last year's minor premiers as they went to 1-5 for the season.
Roosters coach Trent Robinson said second-rower Sio Siua Taukeiaho had suffered a medial ligament injury and was facing several weeks on the sidelines.
Robinson described Napa's performance as exemplary after deciding he was going to dig his side out of their predicament.
"Not many players can do what he did tonight," Robinson said.
"Credit to him - he decided he was going to have a game tonight and he did. A lot of people decide and they can't follow through.
"He delivered."
After halves Cody Walker and Luke Keary failed to make the most of their numerous opportunities, Rabbitohs mentor Michael Maguire lamented his side's lack of direction.
Halfback Adam Reynolds is expected to return from a broken jaw next week however Maguire said his presence wouldn't be a panacea for their problems.
"There were times when we had control in the set and we didn't take opportunities. We looked like we were going sideways - there was no direction," Maguire said.
"We'll have to have a look at that. We've got two halves who are capable of doing that and we didn't get that done today."
- AAP
