Teams name a 41-man squad before the tournament starts and flanker Phillips was not included in the original party.

Scarlets have seven back-row injuries ahead of next week's match against Leicester but cannot register Phillips.

"It is an archaic system that needs to be looked at," said Pivac.

Each club may register up to three additional players during the pool stage to replace a player previously selected but Scarlets have used up their allocation.

Pivac's side are out of contention for quarter-final qualification having lost their first four matches but wanted to use Phillips in the final two matches against Leicester and Racing 92.

Wales hooker and Scarlets skipper Ken Owens had to play at number eight for the 22-13 win against Dragons after packing down alongside young flankers Ed Kennedy and Dan Davis.

Ken Owens played for the Scarlets for the first time at number eight against Dragons

Phillips was on the replacements bench after just returning from concussion but Scarlets wanted to start the former Wales Under-20 captain in Europe and not have to play Owens out of position again.

The region has an extensive back-row injuries list including James Davies, Aaron Shingler, Will Boyde, Blade Thomson, Josh Macleod and Uzair Cassiem.

Pivac believes the policy is threatening player welfare.

"Unfortunately, Tom can't play in Europe because we have used all our injury cards," said Pivac.

"I understand why they do it but this is a guy that is ranked in our loose forwards and has been in the club all his life.

"He can't play despite us having injuries so we have to play players out of position and put them at risk.

"We have already asked the question in the last round but we were turned down so there is no point going there again.

"It is something at the end of the season we will definitely discuss."