Wales wing George Gasson will not travel with the team to the World Rugby U20 Championship, which kicks off in Manchester next Tuesday.
Gasson sustained a hamstring injury in training. He is replaced by Vale teenager Jared Rosser.
The talented Rosser made his debut for the Steelmen as a then 17-year-old this season. He scored the decisive try in their Premiership play-off semi-final victory against Llandovery and also went on to be a member of the victorious Vale side which swept aside Pontypridd in the Premiership final at Sardis Road.
Wales U20 team manager Mark Taylor, said: "It's obviously disappointing to lose a player of George's ability so close to the tournament but Jared is now given an opportunity and his selection is reward after a fine season with Vale."
Wales take on Ireland, Georgia and New Zealand in Pool A of the global tournament, with Six Nations opponents Ireland first up at the Manchester City Academy Stadium.
A Vale spokesman added: "All at the club are delighted for Jared, it is a major achievement to make the U20's World Cup for someone who has only recently turned 18.
"It is reward for the hard work he has put in and the coaches at Vale have been delighted with his progress on the field and his attitude and behaviour off it. This is a credit to himself, his family and the Dragons Academy."