The former Groclin Dyskobolia player joined Derby in September 2004 after a transfer to Italian Serie A side Siena was not registered with the football authorities and George Burley nipped in quickly.
The move to Pride Park proved a great success and the striker netted 17 goals as Derby reached the play-offs in May 2005. That earned the powerful 6ft 3" frontman a £2m move to Spurs.
However it did not work out and in February 2006 he joined Saints on loan to the end of the season with a view to a permanent deal. That was completed in May with Saints paying Tottenham £2m for the Pole who signed a four-year deal before linking up with the national squad for Germany 2006. He made one brief substitute appearance in the final group game when Poland were already eliminated.

He made a blistering start to the 2006-07 campaign with 10 goals in the first 10 league games and 21 by January before fellow Pole Marek Saganowski took over the goalscoring mantle. Rasiak though bounced back with the crucial late goal to force extra-time in the play-off semi-final at Derby, taking his tally for the season to 21 in all competitions.

He began the 2007-08 campaign in fine form netting four times in the first five games. In January 2008 he joined Bolton on loan but just failed to win a place in Poland's Euro 2008 squad.

Grzegorz spent the 2008/09 season on loan at Championship side Watford, where he scored eight times in the league.