Manly are resigned to losing inspirational co-captain Jason King for the rest of their NRL campaign, with their 36-8 thrashing of the Sydney Roosters coming at a massive cost.

The cornerstone of the Sea Eagles' bruising pack, King will have scans on a pectoral muscle injury and the club will learn the full extent of the problem on Monday.

The NSW State of Origin representative left the field after 20 minutes of the Brookvale Oval clash and officials said he was shattered after learning he would not be a part of Manly's tilt at the premiership.

"It doesn't look too good," coach Des Hasler said. "We'll know more tomorrow after the scan.

"They're pretty scary, those sorts of injuries.

"It would be a real shame (if he was ruled out for the season).

"He's been one of our most consistent but somebody else will have to step in and do the job."

The Sea Eagles were close to full strength a month out from the play-offs, and Hasler sought to dispel concerns over two-try duo Will Hopoate (hamstring) and Steve Matai (neck), who also appeared to suffer injuries in the eight tries to two mauling.

Hopoate's leg was iced when he left the field late in the match, while Matai lay on the turf for several minutes in the dying stages of the game before exiting the field clutching his neck.

Hasler described Hopoate's benching as precautionary, while key forwards Anthony Watmough and Glenn Stewart were taken off for a rest when the match was in the bag.

King's front row partner Brent Kite said the rugby league public were only just starting to realise what a massive presence he is for the Sea Eagles, who firmed up second spot behind Melbourne with Sunday's ruthlessly-efficient win.

"Perception's a big thing and him making rep teams, he's just started to do that in the last couple of years," Kite said.

"He's a very tough guy and maybe, as a result, he picks up these extra little injuries at the wrong time.

"You both get a stat for a tackle but Kingy's dead-set rattled the other bloke."

Kite said he and less-heralded forwards like George Rose and Tony Williams were aware they would need to step up.

"We've missed Kingy one or two games this year and, as a group, we've been aware the intensity that he does bring can be missed if we're not picking up the slack," Kite said.

It was a case of four footy seasons in one day as lightning and sunshine took turns to light up Brookvale but it was the Sea Eagles who shone brightest in front of 13,745 fans.

They led 12-0 at halftime and the damage would have been worse for the Roosters if Jamie Lyon had managed better than two from eight with the boot.

"They settled into a routine," Roosters coach Brian Smith said.

"Their precision and their execution and the quality of their defence was way too good for us."

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