[2] Two years later, Nielsen and Vestsjælland reached promotion to the Danish Superliga and he made his debut in the highest tier on 21 July 2013 in a 1–1 draw away against Brøndby IF.

[5] His performances during the 2013–14 season attracted interest from various Superliga-clubs and on 14 June 2014, Nielsen signed a two-year contract on a free transfer with OB.

[7] On 18 May 2016, with Nielsen's contract expiring in OB, he was signed by Polish Ekstraklasa club Lech Poznań on a three-year deal.

[8] On 7 July 2016, Nielsen won the Polish SuperCup in a 4–1 win over Legia Warsaw, making his first start and scoring his first goal for the club.

[9][10] He made seven starts in his first 15 league games in Poland, and called the level on which he played a clear "step up" compared to the Danish Superliga.