Cuthbert returns as Wales hand debut to Ospreys flanker against Scotland
Morgan, 22, replaces Ellis Jenkins and lines up at openside flanker in a reshaped back-row that also sees Ross Moriarty start at number eight instead of Aaron Wainwright, with Taine Basham wearing the number six shirt.
Elsewhere, Morgan’s Ospreys colleague – wing Alex Cuthbert – makes a first Six Nations appearance for five years, being preferred to Johnny McNicholl.
But Cuthbert’s fellow wing Josh Adams, who started at outside centre in last Saturday’s Six Nations opener against Ireland in Dublin that Wales lost 29-7, is ruled out because of a tight calf muscle, so Owen Watkin fills the number 13 position.
Pivac has resisted any temptation to make further changes, while captain Dan Biggar makes his 100th Test match appearance for Wales and the British and Irish Lions, becoming the ninth Welshman to achieve that feat.
Biggar wins a 97th Wales cap, with Wainwright among the replacements, but McNicholl and Jenkins, who skippered Wales in Autumn Nations Series games against Fiji and Australia this season, drop out completely.
Scotland have not won in Cardiff for 20 years, losing eight Six Nations Tests, a World Cup warm-up game and an autumn international during that time.
But they will arrive at the Principality Stadium following a stirring Calcutta Cup victory over England last weekend that confirmed serious Six Nations title credentials.
Wales, meanwhile, have not started a Six Nations campaign with successive defeats since 2007, and they must beat Scotland to revive any hope of a successful title defence.
On Biggar and Davies, Pivac said: “To get 100 Test matches in total for any player from any country, it’s just a magnificent achievement.
“It shows the hard work and dedication they have put in and the sacrifice they have made over a number of years to get to this stage.
“I am very, very happy for both of those players to achieve it and to do it on the same day.
“They have played a lot of Test matches together in the past. It is going to be fantastic to see them both out there at some stage.”
Moriarty won his 50th cap off the bench in Dublin, and he now makes a first Test start since suffering a shoulder injury during Wales’ defeat against New Zealand on October 30.
Pivac added: “With Ross now having had more training and game-time under his belt, we think he is the right guy to start.
“Jac Morgan comes in for his debut. He is someone that has really impressed us in training. He has worked very, very hard, he gives us a lot of physicality and he is very good over the ball.
“We expect Scotland to come down full of confidence. Across the board, they are a very competitive and committed side that throws everything at every play.”
Wales team to play Scotland in Cardiff on Saturday.
L Williams (Scarlets); A Cuthbert (Ospreys), O Watkin (Ospreys), N Tompkins (Saracens), L Rees-Zammit (Gloucester); D Biggar (Northampton, capt), T Williams (Cardiff); W Jones (Scarlets), R Elias (Scarlets), T Francis (Ospreys), W Rowlands (Dragons), A Beard (Ospreys), T Basham (Dragons), J Morgan (Ospreys), R Moriarty (Dragons).
Replacements: D Lake (Ospreys), G Thomas (Ospreys), D Lewis (Cardiff), S Davies (Cardiff), A Wainwright (Dragons), G Davies (Scarlets), C Sheedy (Bristol), J Davies (Scarlets).
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