But captain Daly Cherry-Evans insists he is still "very confident" Manly can play finals despite slipping down to 10th spot with the defeat at a wet and wild Brookvale Oval on Saturday night.
The Roosters could've all but kissed their own top-eight dream goodbye with a loss to fellow top-eight hopefuls Manly, having dropped three of their previous four matches.
But in a duel against Cherry-Evans, his probable halves partner next year, No.7 Walker breathed life into his side's season by playing a hand in all three Roosters tries.
Captain courageous James Tedesco set the tone for the Roosters' best defensive performance of the year with two try-saving tackles on the right edge while the game was in the balance.
The Tricolours have now leapfrogged Manly into ninth on the ladder, a win out of the top eight and seemingly locked in a three-way battle with the Sea Eagles and Dolphins for eighth place.
"People always talk about the external, the ladder and all that but it doesn't matter if you're not playing your best footy," said Roosters coach Trent Robinson.
"The first one is you've got to acknowledge how do we play our best footy? But there is also the acknowledgement that we can't slip up there if we want to have a crack in September.
"That (win) was what we needed."
The loss may come back to haunt Manly given it keeps their rivals in the hunt and leaves them likely needing to win at least two tough upcoming clashes against Canberra, the Dolphins and the Warriors.
"The belief is still there," said Cherry-Evans of Manly's finals hopes.
"We just need to keep giving ourselves a chance and at the moment, the last two weeks in particular, we haven't given ourselves a chance.
"From where I'm sitting, I'm very confident we can still get there. It could take until the last round but that's all you need."
Manly enjoyed the bulk of early chances but had only one try to show for it as Tom Trbojevic, playing fullback during Lehi Hopoate's head injury assessment, swung left to put Tolu Koula over.
Hopoate returned but was shifted to the wing late in the game with Trbojevic slotting into fullback after weeks of debate as to how Manly should best use the former Dally M Medallist.
Terrible conditions played their part as Trbojevic spilt the ball into the hands of Queensland star Robert Toia, whose charge upfield swung momentum dramatically into the Roosters' favour.
On the back of that Toia line break late in the first half, Walker found Billy Smith with a grubber kick for the Roosters' first try and the visitors were over again in the next set.
Tedesco belted through the middle past Jake Trbojevic and Tof Sipley and found Walker, who slipped over but regained his footing and kicked left to a diving Daniel Tupou.
The veteran winger managed an offload for Angus Crichton to score one of the season's great team tries.
It was a three-score game when a Walker kick was allowed to bounce into the arms of Egan Butcher at close range early in the second half.
Manly's Ben Trbojevic suffered a game-ending concussion when clashing heads with teammate Luke Brooks, playing his 250th game, and hooker Jazz Tevaga left with a serious hip injury late.
Tevaga, 29, may have played his last NRL game given he will move to Super League side Wakefield Trinity on a two-year deal next season.
"He can't put any weight on his hip, he felt a pop in his hip. It doesn't sound good," said Manly coach Anthony Seibold.
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