The 32-year-old halfback will have to watch Saturday's qualifying final away to Penrith from the stands after pulling up with the issue in training on Wednesday.
With Johnson joining first-choice five-eighth Luke Metcalf on the sidelines, coach Andrew Webster's side are set to turn to rookie Ronald Volkman and journeyman Te Maire Martin for the match against the minor premiers.
The club posted on social media on Thursday to say Johnson would "be available for selection for our next match" after he was spotted leaving Auckland with a support strapping on his left leg.
But in the biggest game in recent Warriors history and against a formidable opponent, Johnson's loss cannot be understated.
The Kiwi international has been key to the Warriors' revival this year and his career-best form has made him the leading contender to take out the Dally M Medal.
Johnson is said to have undergone limited training this week before a call was made soon before the Warriors touched down in Sydney on Thursday.
Webster had batted away questions on Wednesday when probed on his halfback's fitness.
Reporters in Auckland noticed Johnson wasn't taking part in a warm-up session.
"That's happened several times this year," Webster told reporters then.
"We do extended warm-ups so Shaun warmed up on field No.1 today (Wednesday).
"We go straight into collisions and contact and we just manage that and time it better.
"I knew I was going to get asked that. I don't care if I get asked that because it's literally happened so many times."
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