A central defender who was 19-years-old when he joined from CS Fola Esch in Luxembourg in May 2017.
Although born in Pec, FR Yugoslavia, Enes plays his international football with Luxembourg and already has five caps for the national team.
He had won four caps with Luxembourg U21s before making his debut for the full team in a World Cup qualifier in November 2016.
To date he has five full caps, the most recent, also against the Netherlands in a World Cup qualifier, came in June.
Mahmutovic made his first appearance in a Boro shirt when playing in a friendly game against Oxford United during a training camp in Portugal, playing alongside Daniel Ayala in the first half of a 0-0 draw in Albufeira.