Chester-born Koumas has been in the Liverpool academy since the age of 11. He signed his first Liverpool professional contract in January 2023.
He scored on his senior Liverpool debut in a 3-0 FA Cup fifth-round victory over Southampton in February, having been an unused substitute for Jurgen Klopp’s side in the 1-0 League Cup final win over Chelsea.
Captain and Cardiff midfielder Aaron Ramsey and his usual deputy Ben Davies, of Tottenham Hotspur, are both doubtful with injuries.
David Brooks was injured in Southampton’s 1-0 Championship play-off final victory over Leeds United last weekend.
Page will also bear in mind some of the workload his players have had to endure in the past season, as he announces a squad which very much has the future in mind.
The Wales manager is keen to provide up and coming young players the opportunity in the Wales set-up.
Page said: “We need to expose them to games like we did with (Tottenham forward) Brennan Johnson two years back against Holland and against Belgium.
“He is a better player having gone through that, so that process worked. We want to do the same now with some of the best young players we have.”
After the June friendlies, Wales will kick off their Nations League campaign at home to Turkey on 6 September, before going to Montenegro on 9 September.