Messi had his most successful season of his playing career in which he scored 38 goals to play an integral part in a treble - winning campaign .
His most successful season of his playing career is the 2008 - 09 season .
Messi had his most successful season of his playing career , the 2008 - 09 season , in which he scored 38 goals to play an integral part in a treble - winning campaign .