Team doctor John Mayhews initial assessment has been confirmed by orthopaedic surgeon Stu Walsh.
Mayhew said an MRI scan has revealed a complete rupture of the right pectoralis major muscle. Its expected the 22-year-old Fisiiahi will be operated on in about 10 days.
"The usual recovery period is in excess of three months following surgery, said Mayhew.
Glen wont be available for further football this year.
The injury means Fisiiahi not only misses the rest of the Vodafone Warriors 2013 NRL campaign but is also ruled out of playing for Tonga at the Rugby League World Cup in October-November.
Its terrible luck for Fish and a real setback for us, said Vodafone Warriors head coach Matthew Elliott.
He had been doing a terrific job for us. Naturally hes devastated and we all feel for him. What well do as a group is give him all the support we can to ensure he recovers well for next season.
Fisiiahi, who has had his best season since his NRL debut in 2011, left the field in the 60th minute of the clubs 24-14 win over Wests Tigers.
After making eight appearances in 2011 and 2012, he has more than doubled his career tally to 19 matches, scoring five tries in his 11 games this year. He also made his Test debut for Tonga in April.
Expected to take Fisiiahis spot on the right wing is 20-year-old Ngani Laumape, who has made eight NRL appearances since his debut against Cronulla in March.
He becomes the fourth Vodafone Warrior with a season-ending injury. Sione Lousi was ruled out for the year with an ACL knee injury in the first trial of the season in February while Steve Rapira and Ben Henry followed later when they also needed surgery to repair ACL injuries.
Manu Vatuvei (shin) and Pita Godinet (knee) both made successful returns from injury last weekend, Vatuvei making 112 metres from 12 runs in a strong display against the Tigers on Friday night and Godinet playing his first game in a month for the Auckland Vulcans against Newtown in the New South Wales Cup yesterday.
Apart from Fisiiahi, Lousi, Rapira and Henry, the Vodafone Warriors have Russell Packer (hamstring) and Charlie Gubb (groin) on the injured list. Its hoped theyll be available for selection this week.
The current injury status report is:
Glen Fisiiahi pectoral (out for season)
Charlie Gubb groin (back for round 20)
Ben Henry knee (out for season)
Sione Lousi knee (out for season)
Russell Packer hamstring (back for round 20)
Steve Rapira knee (out for season)