The Tigers feared Koroisau's season would be over after he was taken off in Saturday's shock loss to wooden-spoon favourites St George Illawarra.
Koroisau was sent for scans on Sunday knowing that he would be ruled out the rest of the season and Fiji's Rugby League World Cup campaign if he required surgery.
But the results showed Koroisau had only suffered a pectoral strain rather than a tear, giving the 33-year-old No.9 hope of returning to NRL action this season.
The Tigers indicated Koroisau would be sidelined for the next month.
"Api Koroisau will be out for four weeks with a pectoral muscle strain,'' the club said in a statement.
"Scans taken on Sunday have revealed a low-grade strain of the muscle tendon junction following Saturday night's game against the Dragons."
The chance of Koroisau returning before the season's end is a major result for Benji Marshall as he attempts to end the Tigers' 15-year finals drought.
The Tigers (7-9) sit 12th following their 24-10 defeat at the hands of the Dragons and have won just one of their past five games.
Marshall indicated after the Dragons loss that five-eighth Jarome Luai (concussion) would return for Friday's meeting with the Warriors at Campbelltown.
Back-rower Samuela Fainu is rated a chance to make his comeback from a foot injury against the Kiwi outfit.
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