Alena has long been circled as a potential star of the future after coming through at Barca’s La Masia youth academy but he has been unable to establish himself in the first team.
He failed to make much of an impression during a loan spell at Real Betis last season and has made only five appearances for Barca this term under new coach Ronald Koeman.
The Spaniard scored on his debut in the Copa del Rey in November 2016.
“Alena is a player for the future that has to play,” said Koeman on Tuesday. “He has decided to look for an exit.”