Flanker Thomas Young will join Cardiff next season from Wasps to link up with his father Dai and is available immediately for Wales selection.
The 29-year-old has been given special dispensation to the Welsh Rugby Union's (WRU) 60-cap rule.
Wales play New Zealand, South Africa, Fiji and Australia this autumn.
The WRU introduced a policy in 2017 called the Senior Player Selection Policy (SPSP) that deemed if you committed to playing for a club outside of Wales and had fewer than 60 international caps, you were ineligible for the national side.
It has resulted in players joining or returning to one of the four regions, but has also hampered the international ambitions of others like Young and limited Pivac's options.
"It was something that has definitely been forced upon us," Pivac admitted.
"Under the 60-cap rule we knew we couldn't select Thomas. We've had six openside flankers go down with injury and be ruled out.
"On the weekend just gone, we went to watch Josh Navidi and he went down on the Saturday night. Then we were at the Scarlets game on Sunday and another seven, Dan Davis, went down. It's been a perfect storm in that position.
"Thomas has form under his belt, playing in a strong league. He's played international rugby for Wales before. He's 29 years of age, he's got the maturity.
"When you look at the opposition, we felt it was special circumstances for this particular competition. We've asked for the ability to select him and I'm very happy to say common sense has prevailed on this occasion."
As he will continue to play his club rugby this season in England, Young will not be available for the 30 October game against New Zealand as it falls outside World Rugby's official international window - meaning Wasps need not release him for that fixture.
However, Young will be available for the following games against South Africa, Fiji and Australia at Cardiff's Principality Stadium.
There has been one previous exception for a player who indicated he was returning to a Welsh region the following season.
Scrum-half Rhys Webb was allowed back into the international fold for the 2020 Six Nations when he announced he was returning to Ospreys from Toulon for the 2020-21 season.
A similar move to allow Rhys Priestland to play for Wales last season, after it had been announced he was linking up with Cardiff from Bath, was rejected.
The dispensations are decided by the Professional Rugby Board (PRB), which runs the professional game in Wales and is made up of representatives of the regions and WRU.
"We've asked on three occasions and on each occasion we've felt it's been necessary to get the performances we need to help the team. This is no different," Pivac added.
Young won his first cap in June 2017 against Tonga and his third and last came against Italy in the 2019 Six Nations victory in Rome. Having been part of the Grand Slam squad that year, Young has not figured for Wales since.
Young left Gloucester to join his father's Wasps team in 2014 and when it was announced he had committed to stay at Wasps in 2020 after his father had left, the back-rower made himself ineligible to play for Wales.
That has now disappeared following the confirmation of him returning to Cardiff for the 2022-23 season where he links up with his father Dai, who has also come back for a second stint at the Arms Park.
"I'm excited to have this opportunity at Cardiff and looking forward to returning home to Wales," said Thomas Young.
"It was a difficult decision to make because I've had an enjoyable time at Wasps and learnt so much, which I remain grateful for.
"I'm also a proud Welshman and the opportunity to stake a claim for further international honours was too great to turn down.
"Once I made that decision, Cardiff was always my preference - this is my home club. I played all of my age-grade rugby for the region and have a lot of friends here from that time.
"I'm looking forward to linking up with the boys and contributing next season but remain fully committed to Wasps and want to sign off on a high."
Wales have back-row injury problems this autumn with Justin Tipuric, James Botham and Dan Lydiate ruled out of the international series and Josh Navidi a major doubt after damaging his shoulder against Bulls.
Bath number eight Taulupe Faletau, like Young, is unavailable for the opening match against the All Blacks.
"When Thomas made the decision to return to Wales, we were keen to bring him home to Cardiff - that goes for myself, the coaches and board," added Dai Young.
"It is my job to improve and develop this squad and I have no doubt Thomas will significantly add to our strength and depth in the back-row and drive standards in our environment.
"He suits the style we want to play and I know he will do his talking on the pitch and show everyone in Wales how good he is."