Ricky Stuart says increased awareness of head knocks means having three players ruled out due to concussion could become a more common occurrence.
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And the Canberra Raiders coach feels an 18th man is needed simply to reduce the pressure on club doctors.
Stuart also advocated for the inclusion of an 19th man to allow NRL clubs to have a forward and a back to call on depending on who was ruled out.
The Raiders voiced their views to the NRL on the rule change on Thursday, which will see an additional player sitting on the bench from the start of round five.
It has strict regulations around it, with three players having to fail their head injury assessments before the 18th man could be used.
That player also had to be an emerging player, which the NRL was yet to define.
But Stuart said they should be the best players available and the rule should also be invoked when a player was lost due to foul play.
While Stuart was a fan of bringing fatigue back into the game, he felt the concerns around head injuries meant it was necessary to have an 18th man.
"I'm a coach who loves the attrition. We train to play under fatigue," he said at the Raiders' captain's run on Friday.
"We've introduced fatigue into the game, but the concussion HIA laws produces a completely different scenario because we're looking at litigation, we're looking at trying to protect the players and I really feel that from a political correctness point of view our doctors are under so much more pressure when a player comes off.
"I'd be happy with staying with four reserves, but it's going to happen so let's try to have everyone's input.
"It's great the league are asking every club their opinions and they'll throw all that information into the think tank and they'll come out with the right process.
"We didn't realise it's going to be so much influence and pressure on our doctors of basically keeping out players after an HIA - because that's what we're seeing at the moment.
"We might be closer to three HIAs a game more so than we ever have in the past."
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He felt there were better options than having an emerging player as the 18th man.
"The NRL are talking about an emerging player because they're concerned about coaches exploiting the rule," Stuart said.
"We need to look at what's going to be best for the game. Is it the next best player?
"We spoke about having an 18th and 19th man - having a middle and a half or a middle and an outside back on your bench.
"And you can use only one of those, but use one in making the decision on who got injured.
"[It's] no good replacing an outside back with a middle just for the sake of it. We want to keep the quality of the game at a high.
"I don't see there being any reason why we can't have two sitting on the bench with one ready prepared to play. The NRL will get to an answer pretty quickly."
NRL ROUND FOUR
Saturday: Canberra Raiders v Gold Coast Titans at Kogarah Park, 7.35pm.
Raiders squad: 1. Charnze Nicoll-Klokstad, 2. Bailey Simonsson, 3. Jarrod Croker (c), 4. Sebastian Kris, 5. Jordan Rapana, 6. Jack Wighton, 7. George Williams, 8. Josh Papalii, 9. Josh Hodgson (c), 10. Dunamis Lui, 11. Hudson Young, 12. Elliott Whitehead, 13. Ryan Sutton. Interchange: 14. Tom Starling, 15. Siliva Havili, 16. Sia Soliola, 17. Ryan James. Reserves: 18. Sam Williams, 19. Semi Valemei, 20. Caleb Aekins, 21. Emre Guler.
Titans squad: 1. Alexander Brimson, 2. Anthony Don, 3. Phillip Sami, 4. Patrick Herbert, 5. Corey Thompson, 6. Tanah Boyd, 7. Jamal Fogarty, 8. Jarrod Wallace, 9. Mitch Rein, 10. Tino Fa'asuamaleaui, 11. Kevin Proctor, 12. David Fifita, 13. Tyrone Peachey. Interchange: 14. Moeaki Fotuaika, 15. Sam Lisone, 16. Jaimin Jolliffe, 17. Erin Clark. Reserves: 18. Beau Fermor, 19. Herman Ese'ese, 20. Sam McIntyre, 21. Brian Kelly.