Mertens made his Belgium debut in , and made over appearances for the national team. In , he was named Belgian Footballer of the Year. Born in Leuven , Mertens started his career as a boy at the now-dissolved local club Stade Leuven.
He spent five years in the youth setup at Anderlecht before being released in , with coaches finding him to be too short and physically incapable of competing at a professional level. Following two seasons in Gent's youth academy, Mertens was loaned to Eendracht Aalst for the —06 season. He excelled with the third division side and was named Player of the Year.
It was a move that paid off for the Dutch club as Mertens ended the season having 15 goals in 38 appearances for the club. Such was his influence that Mertens was named new club captain for the —09 Eerste Divisie season. Mertens made appearances across all competitions for AGOVV during his three seasons at the club and netted 31 goals.
It was revealed on social media platform Twitter that Pasveer had found one of Mertens' teeth lodged in his head three weeks after the players had collided. He also aided his side's cause with a further 17 assists. Mertens claimed another winners' medal on 22 December after Napoli's penalty shoot-out victory over Juventus in the Supercoppa.
It also saw him become the first player to score successive league hat-tricks since Juventus' Pietro Anastasi in , and the first to score seven goals across two matches since Antonio Angelillo in He was later substituted early in the second half due to injury.
