Born in Heemskerk, North Holland, Numan started his career with the amateur club SV Beverwijk and was spotted by Haarlem, who handed him his first professional game against DS '79 on 26 March 1988 which his team won 2–0.

He originally played in a more attacking role, but his long-time mentor Dick Advocaat, then the coach of Haarlem, positioned him as a left fullback.

A £4.5 million deal took him to Rangers in May 1998, but Numan's time in Scotland was then blighted by injuries that forced him to sit out many games.

Numan continued to feature in the Dutch national team, representing his country on home soil during the Euro 2000 tournament.

By 2000, Numan's career at the international stage appeared to wane and his automatic left back spot was open to his contenders Winston Bogarde and Giovanni van Bronckhorst.

He was to play for the remaining games of his team's failed attempt to capture their first major trophy since 1988 culminating in the semi-final loss to Italy on penalties.

He was determined to assist his national team to qualify for the 2002 World Cup in South Korea and Japan under manager Louis van Gaal but the crucial game against Ireland at Landsdowne Road stadium in late 2001 was lost 1–0.