Garrick was named at fullback for Saturday's NRL clash with Cronulla, but scans have since shown an injury in his shoulder.
The stand-in fullback will miss the rest of the NRL season, with the Sea Eagles already out of finals contention following their poor loss to the Gold Coast.
The injury leaves Manly well short on backs.
Garrick had only been playing fullback in place of Tom Trbojevic, who had his own season ruined by a shoulder injury in May.
Winger Jason Saab is also out for the season with an ACL rupture, after the Sea Eagles released veteran flyer Jorge Taufua to the English Super League.
The news is likely to leave Manly needing to bring a player from outside of their 21-man squad into the team, with no outside backs listed.
Reserve-grade winger Raymond Vaega is also suspended, leaving the Sea Eagles low on troops.
It means Alfred Smalley could potentially earn a second NRL match, after debuting in their rainbow jersey match against the Sydney Roosters earlier this month.
Manly were already stretched elsewhere with Dylan Walker to start at lock for the first time in his career with Jake Trbojevic also out for the season with a hand injury.
The Sea Eagles have lost their last four matches to fall out of finals contention, with players having to deny claims of disunity in the fall out of the seven players sitting out the Roosters match.
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