The 19-year-old, nicknamed the 'Balkan Messi' after Argentina forward Lionel, joined Barca from Dynamo Zagreb last year and made 30 appearances for the B side, scoring four goals.
He made his first-team debut in January in a King's Cup match against Elche and also featured for Luis Enrique's outfit on their recent pre-season tour of North America.
He became the youngest player to feature for Croatia in June 2013 a week before his 17th birthday and was the youngest to play for Zagreb the previous year.
(Reporting by Iain Rogers, editing by Ken Ferris)
