North Queensland fullback Lachlan Coote faces a season on the sidelines after suffering a suspected ACL tear at the NRL Nines in Auckland on Saturday.
Coote was one of a series of injuries on the opening day of the sold out Eden Park event.
Wests Tigers confirmed utility Curtis Sironen will miss six weeks after scans revealed a fracture in his right foot.
Sironen suffered the injury early in the first half of the Tigers' win over Cronulla.
Sharks playmaker Todd Carney (hamstring) and Sydney Roosters prop Isaac Liu (knee) will play no further role in the tournament, while Gold Coast back-rower Dave Taylor (knee) is struggling to be fit for the final day on Sunday.
But it is Coote who suffered the cruellest of fates. He limped off in the seventh minute of the Cowboys match against Manly early in the day, clutching his knee after being brought down after a long run.
The former Penrith fullback was taken to hospital with scans, with Cowboys officials fearing the worst - a season ending injury.
"He is one of our key signings and for him personally, he tore his pec last season, if this is a season ending injury it is tragic," new Cowboys coach Paul Green said.
Coote was picked up this season by the Cowboys as a replacement for club legend Matt Bowen, after missing much of last season at Penrith due to a pectoral muscle injury.
The injury overshadowed what was largely a successful day for the NRL before around 46,000 fans in ideal conditions for rugby league.
The hometown Warriors lived up to their status as pre-tournament favourites to enjoy wins over Canberra and Manly to stamp themselves as the team to beat.
Canterbury, Canberra and St George Illawarra can't progress to the final stages of the tournament after losing both of their pool games.
The Warriors, with Shaun Johnson and Sam Tomkins dominating, Penrith and Parramatta remain undefeated.
Chief executive Dave Smith said the NRL was overjoyed at how the event had been received.
"The day has been a great success," Smith said.
"It has been a great atmosphere, it has been a great spectacle and the football has been amazing."
Smith nominated NRL wooden spooners Parramatta's first-up win over premiers Sydney Roosters as his highlight of the day.
The Warriors created the greatest interest and support while comeback king Brad Fittler created one of the more memorable moments of the day, scoring a 55m try in Sydney Roosters win over Brisbane that kept them in the competition.
The Nines continues on Sunday with each team's last pool game followed by the finals.