He signed a two-year contract and looked to compete for the starting goalkeeper spot with fellow Dane Jan Budtz.
One of the tallest players in the league, Nielsen was known as "The Gentle Giant" of football.
He did appear on the pitch after a match against Manchester City, talking to Jan Budtz after the horrible scenes when Doncaster's first choice keeper Andy Warrington broke his right leg.
Nielsen was eventually released from his contract in October 2006, and moved back to Denmark to play part-time football with Greve IF in the Danish 2nd Division.
This biographical article related to association football in Denmark, about a goalkeeper, is a stub.