On a Saturday night when the Dolphins went third on the ladder and announced themselves as legitimate NRL title contenders at Allianz Stadium, the Roosters fell behind them to fourth.
Sitting first before last week's shock loss to Wests Tigers, the Roosters will now likely need a win against arch-rivals South Sydney on Friday to ensure a top-four finish.
But the bigger issue will be the fitness of Walker, who copped a knock from Kulikefu Finefeuiaki's in a tackle in the second half.
Walker has enjoyed the best season of his career, and loomed as a key man come finals after his brilliant State of Origin series.
But his availability is now in doubt, with Roosters medical staff appearing to assess the half's ankle and lower leg.
"I only saw him briefly there before he went for scans," Roosters coach Trent Robinson said.
"We'll deal with that when we get to it."
The Roosters have Daly Cherry-Evans to move into the No.7 jersey and Hugo Savala as a more than capable five-eighth, but Walker's absence would be significant.
Finefeuiaki was cleared of a hip-drop tackle by the bunker, but Dolphins teammate Kurt Donoghoe could be in trouble for a shoulder to the head of James Tedesco.
Donoghoe has a prior on his record, meaning the Dolphins' link man would be facing three games even if only hit with a grade-two dangerous contact charge.
Morgan Knowles was sin-binned for dangerous contact on Robert Toia's leg in a tackle, at one stage leaving the Dolphins down to 11 men.
The incidents overshadowed what was an otherwise dominant night for the Dolphins.
Kristian Woolf's men have already beaten the Warriors this year, pushed Penrith to golden point and flogged the Roosters during the Origin period.
But this was by far their best performance, and a top-two finish and two home finals is now a mathematical possibility ahead of next week's clash with Gold Coast.
Kodi Nikorima and Isaiya Katoa combined perfectly as they went from 6-0 down to 20-6 up in the final 20 minutes of the first half, while their backline is firing.
Herbie Farnworth outleapt Tedesco for one try, while Katoa and Nikorima linked up perfectly for two in a row in the first half.
The best try of the lot came from a scrum play, when Hamiso Tabuai-Fidow batted on a Nikorima ball to Selwyn Cobbo for one of the winger's two tries.
Nikorima also managed to isolate Toia in defence for another try, while Jeremy Marshall-King fooled Tedesco out of dummy-half for one in the second half.
The Roosters briefly threatened a comeback with the Dolphins down to 11 men, but while Salesi Foketi scored once, Tommy Talau bombed another under terrific scramble from the Dolphins.
"It was our bravest win, and we certainly had periods where we were very complete," Woolf said.
"I'd have to have a look at why we ended up with 12 and 11 men, but to hang in the way we did under that pressure (was excellent).
"It's got everything you want about a team, and everything you want about a team going into finals."
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