The Rugby League Players Association is backing moves for the introduction of an 18th man to prevent teams from being "severely disadvantaged" if they lose a player with concussion caused by foul play.

The issue of teams having an 18th man to draw on has resurfaced in the wake of the NRL's new concussion guidelines, which require any player suspected to have suffered a head knock to be taken from the field for assessment.

Canterbury were left with 16 men in their opening round loss to Brisbane after Mitch Brown was forced out of the game by a high shot, while Roosters officials were questioning how their team could have been disadvantaged if Jared Waerea-Hargreaves had been concussed in the tackle for which Parramatta forward Darcy Lussick took the early guilty plea and accepted a four-match ban on Tuesday.

Under the new NRL rules, teams are entitled to a free interchange if a player is cleared to return within 15 minutes or suffers an injury from foul play but the Bulldogs were still a player short, while Roosters coach Trent Robinson was dismayed that Lussick wasn't sent off.

Both the RLPA and NRL club doctors pushed for the introduction of an 18th man as part of the new concussion guidelines but the idea was rejected by the NRL rules committee.

However, the players' body continues to support the proposal and RLPA president Clint Newton said teams were unfairly disadvantaged if they lost players due to foul play.

"It has been discussed but for now we are very pleased with the new concussion rule and fully support this change," Newton said.

"Without knowing what the NRL's preference is on the use of an 18th man I do believe if a team loses a player for the remainder of the game via foul play from the opposition then that team is severely disadvantaged."

RLPA player welfare manager Michael Crocker said he was unsure if concussion should be treated differently to any other injury - except in cases of  foul play.

"I was speaking to Clint Newton about it and if it is an illegal play which ends with a concussion and the player doesn't come back that is when we can utilise the 18th man and he can come on so the team doesn't lose a player through foul play," Crocker said.

The RLPA's submission to the NRL proposed that a player outside the 17-man squad for the match sit on the bench and could be called on as a free interchange while a player was being assessed for concussion.

The 18th man would then be replaced if the player was cleared to return or would take his place if he was ruled out of the match.

It has been suggested the 18th man could be a player who has already had a match for his club's under 20s or in the NSW Cup.

RLPA's Gennie Sheer said the body had not changed its stance towards the introduction of an 18th man and supported the proposal for all cases of concussion - not just those caused by foul play.

Meanwhile, the NRL is investigating possible breaches of the concussion rule in the opening two rounds by Parramatta and Penrith.

The Eels have been asked for a report on why Will Hopoate was allowed to remain on the field in their opening round win over the Warriors when he appeared dazed, while the Panthers have also been asked to explain the delay in replacing hooker James Segeyaro during last Saturday's match at Melbourne.

There has been some confusion around the rules in the opening weeks, with Broncos officials unsure if Todd Lowrie could return to the field after a concussion, and the Roosters replacing a different player after Frank Paul Nuuasala was cleared than the one who had come on for him.