With just one point from their opening two European fixtures so far this season, only victory will do for Scarlets if they want to keep their campaign alive, but Pivac is not prepared to take chances with the likes of Barclay or Shingler with a long way still to go this season.
"We are the walking wounded at the moment and we have some big games to come," Pivac told the South Wales Evening Post.
"We had a lot of injuries last year and I thought that was a lot, but it just seems to be commonplace, that is something I am learning.
"We have got to be very careful because it is a long season. You have to listen to people's bodies.
"Aaron and John's form has probably dipped away in the last couple of weeks and that is down to the workload they have had, they have played a lot of rugby and are genuinely knocked about.
"There have been a lot of minutes for certain players and it has taken its toll.
"Last weekend, John Barclay was relishing the opportunity to get back into the seven jersey, but the body wasn't willing. Aaron Shingler, the same, we were going to take him off at half-time, it happened just after half-time, there was just nothing in the tank.
"You have got to put the team first. We have got some guys that are fresh and ready to go and we believe they will give us more than a 60 percent John Barclay or a 50 percent Aaron Shingler."
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