Coote limped off in the seventh minute of the Cowboys match against Manly on Saturday, 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.
If confirmed, the injury would continue a luckless run for Coote who missed much of last season for the Panthers due to a pectoral muscle injury.
Coote was picked up this season by the Cowboys as a replacement for club legend Matt Bowen, who wasn't offered a new contract and was subsequently picked up by Wigan.
Sydney Roosters prop Isaac Liu was another tournament casualty.
Liu suffered a knee injury in the Roosters opening game loss to Parramatta and won't play again in the tournament which concludes on Sunday.