Johnson's try was converted to level the scores 6-6 but the shell-shocked Warriors were steamrolled by Manly in the second half as the visitors ran out 32-12 winners.
NRL: Warriors suffer their worst loss
McFadden believes he may need it operated on and is likely to miss the Kiwis end of year tour to England. McFadden admits it's the worst possible scenario.
"It's not good for Shaun. It looks like it's going to be season ending so he's gone to hospital now and I think he may have to get operated on," McFadden said.
"I'm not quite sure of the full extent of the injury but it's probably on the worse side of injuries so it's very disappointing for us but those are the things you've got to deal with."
The question now remains as to what exactly the Warriors do without their star halfback, who had been in arguably the best form of his career.
Tui Lolohea moved from centre to play alongside Chad Townsend in the halves last night, with Sam Tomkins also lending a hand from fullback.
"It was a really tough night," Tomkins said. "We train every week with players in different positions, so that's no excuse.
"I've played in games where there's been plenty of stoppages because of injury but tonight was very tough. Our defence wasn't good enough. Shaun didn't concede 32 points and, with the score at 6-6 at halftime, we were right in the game.
"We've got a big squad and somebody is going to have to step up now. There are a few guys who can take on those responsibilities."
The game wasn't pleasing on the eye and disrupted at regular intervals as Johnson's injury was just one of many.
Bodene Thompson left the field in the second half, also with an injury to his left ankle, but it doesn't appear to be as serious as Johnson's because he was walking around after the game with a protective cast.
Raymond Faitala-Mariner left the field with concussion in the first half but was given the all clear by the medical team and returned to action.
Manly's Steve Matai also suffered a heavy impact to his head when he fell to the ground in the first half and was wobbling as he was helped off.
Luke Burgess was another who looked in trouble when he landed awkwardly in a tackle and injured his leg and was walking around with a heavy limp after the game.
On top of that, Manly's Ligi Sa (hamstring) and Tom Symonds will need time on the treatment table.
As McFadden trudged back to a subdued dressing, his head down and shoulders slumped, it was clear he knew what Johnson's injury meant.
A bad night, indeed.
