Manu immediately pulled up in pain after attempting to fend off Adam Doueihi during the first half on Sunday night, clutching at his left hand as he left the field.
Early indications were that the rugby-bound star had suffered a fracture, typically ruling players out for at least a month.
His injury was the only sour spot on a dominant night for the Roosters, as James Tedesco starred in his first game since officially losing the NSW No.1 State of Origin jersey.
The fullback had a hand in three tries and scored one of his own, as the Roosters tore the Tigers apart down Manu's right edge in the first half.
The hosts went from 0-0 to 16-0 up in a seven-minute patch without the Tigers touching the ball, with Tedesco particularly damaging through the middle.
He helped create the space for Manu to put Dom Young over for the Roosters' first, before busting through the middle on the play before the winger's second.
Tedesco also batted on a ball for Daniel Tupou to score on the left wing after the break, before breaking through to score his own while the Tigers had Apisai Koroisau and Adam Doueihi in the sin-bin.
Luke Keary also had a good night, scoring one of own and kicking for an unmarked Tupou to score another later in the second half.
The Tigers are now back in last spot and looked nothing like the side that won their previous two matches.
They lacked impetus in attack and only scored points in the final minute, while Aidan Sezer's woeful tackle attempt in the lead up to a first-half Terrell May summed up their night.
Koroisau, Doueihi and John Bateman could also face bans out of the match, for a dangerous throw, hip-drop tackle and high tackle respectively.
But the main concern from the match remains Manu, who is playing in his last season at the Roosters before heading to Japanese rugby.
His injury adds further stress to an already depleted Roosters backline.
Joseph-Aukuso Suaalii has one more game to serve from his State of Origin suspension, while Fetalaiga Pauga is banned for another three games.
Billy Smith is out indefinitely with an ankle injury, while back-up options Robert Toia (knee) and Lewis Murphy (scaphoid) are also sidelined.
The Roosters played Sitili Tupouniua at centre on Sunday night, while Michael Jennings made his return from a hamstring injury via NSW Cup on Saturday.
The club could bring Mark Nawaqanitawase across early, if Rugby Australia grant an early release and a 2024 contract is approved by the NRL by Monday's 5pm mid-season deadline.
But even that may not offer the Tricolours any immediate relief, given Nawaqanitawase could be on rugby sevens duties until August if he makes the Olympics.
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