Born in Arafat in the outskirts of Nouakchott, Hacen started off at local ASAC Concorde and nearly moved to Valencia CF in August 2014 after impressing in the year's L'Alcúdia International Football Tournament,[2] but chose to join Levante UD instead.
[3] Assigned to the club's Juvenil squad, Hacen made his debut with the reserves on 14 January 2016, coming on as a first half substitute in a 1–3 Segunda División B away loss against CD Llosetense.
[5] Hacen made his first team debut on 28 November 2017, starting in a 1–1 home draw against Girona FC, for the season's Copa del Rey.
[8] He made one two-minute substitute appearance for the first team in the top flight the following 23 April, a 1–0 win over Girona FC at the Estadio José Zorrilla.
[16] He also participated in the 2017 Africa U-20 Cup of Nations qualification, where he helped Mauritania reach the second round after defeating Algeria 3–2 on aggregate, scoring two goals in the process.