Saturday's defeat was compounded by the loss of their halfback Sam Walker to a grade one concussion in the seventh minute - ruling him out of next Friday's must-win clash against the Melbourne Storm.
The Roosters are now hanging on at eighth on the ladder, with the Dolphins, Manly and the Wests Tigers breathing down their necks for the chance to play September football.
Despite the difficult circumstances, Roosters coach Trent Robinson is confident in his team's ability to move on from the disappointment.
"We feel like we got some consistency with a team coming back and we started to put together a style of play and, you know, we're not happy tonight. That's really disappointing what happened," Robinson said.
"This is our life, this is what you do. We're going to dust ourselves off and believe in what's possible for us and our style of play and then go down (to Melbourne) and get it done."
It was a game of incredible moments from start to finish, the highlights stretching from Mark Nawaqanitawase's miraculous flick pass behind his back for a Robert Toia try in the 59th minute, followed by Jordan Samrani's crazy four-pointer on the other side of the field set up by a kick from Eels forward Charlie Guymer.
Former Wallabies star Nawaqanitawase also produced a stunning catch off a bomb and offload in a corner tackle for skipper James Tedesco to score.
Nawaqanitawase's flick pass for the Toia try sparked a mini-Roosters comeback, but it was too little, too late for the Tri-colours, who weren't able to overcome several crucial errors.
Parramatta dominated possession and field position all game, finishing with an 85 per cent completion rate to the Roosters' miserable 57 per cent - the lowest effort of any team in any game so far this season.
The Roosters were also forced to make 356 tackles to the Eels' 297, and made an uncharacteristic 21 errors.
The Eels have now won eight games this season, more than last year's seven.
But coach Jason Ryles doesn't see that as a significant milestone.
"I want to win every game," Ryles said. "I keep talking about continuous improvement and getting those little things right every single day, so … it transfers into the games. The scoreboard will be the last thing that changes."
Eels halfback Mitch Moses was integral in the win, leaving fans wondering what the season could have been if injury hadn't hampered him.
It was epitomised by his try in the 51st minute, where he kicked at a perfect angle towards the posts and beat his former teammate and friend Tedesco to the ball to take the score to 20-4.
The Eels put on an almost-perfect first-half performance to go into the sheds with a 10-point lead, highlighted by a pinpoint-perfect kick from lock Dylan Walker – in his 250th game – for Josh Addo-Carr's second try.
The four-pointer was Addo-Carr's 155th career try, which pushes him past Nathan Merritt on the all-time tryscorers list.
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