Knopper playing for Den Haag in 2009
|Date of birth||29 August 1977|
|Place of birth||Rijswijk, Netherlands|
|Height||1.77 m (5 ft in)|
|2002-2003||-> Aris (loan)||21||(1)|
|2003-2004||-> Heerenveen (loan)||32||(9)|
|* Senior club appearances and goals counted for the domestic league only|
Born in Rijswijk, Knopper began his career with Feyenoord but, still in his youth, moved to rivals AFC Ajax, making his official debut in November 1997 against Sparta Rotterdam. In 1999, he was voted by club supporters as Most Talented Player, receiving the Marco van Basten Trophy; the following year he scored 15 Eredivisie goals as a midfielder, but his team could only finish fifth.
A regular under Jan Wouters, Knopper saw his playing time reduce when Co Adriaanse took over as head coach in 2000. He was sidelined with injury for almost the entire season, playing only eight matches that year and appearing slightly more in the following, which would be his last.
In May 2002, Knopper had a trial with Premier League side Sunderland and played in that predicament in the testimonial match for Niall Quinn with the Republic of Ireland on the 14th. From 2002 to 2004 he served two loans, first with Greece's Aris Thessaloniki F.C. then with SC Heerenveen, slightly reviving his career with the latter; upon his Ajax release in summer 2004, he spent two additional campaigns with Vitesse Arnhem, appearing regularly.
After some problems with Vitesse manager Aad de Mos, Knopper was forced to leave and joined ADO Den Haag. In July 2007 a three-year contract was agreed, with the player netting again in double figures (ten) in 2008-09, crucial to help in a narrow escape from relegation; in the 2010 summer, aged 33, he was released.