Near the end of the 2010–11 NHL season, Islanders' Captain Doug Weight was quoted as saying of Nielsen, "He’s our best all-around player even though his stats don’t necessarily show it".

[5] On 25 October 2014, he recorded his first career NHL hat trick against Anders Lindbäck of the Dallas Stars by scoring all three goals in the third period.

[6] On 1 July 2016, Nielsen signed as a free agent to a six-year, $31.5 million contract with the Detroit Red Wings, marking the end of his 10-year tenure with the Islanders.

[11] Unsigned leading into the 2021–22 season, Nielsen signaled the conclusion of his NHL career, returning to Europe and agreeing to a one-year contract with German club Eisbären Berlin of the Deutsche Eishockey Liga (DEL), on 10 October 2021.

Nielsen represented Team Europe at the 2016 World Cup of Hockey, where he recorded two assists in six games and won a silver medal.

Nielsen retired from professional ice hockey after the Danish elimination from the 2022 IIHF World Championship, following a 1-7 defeat to Slovakia.

[15] Nielsen was inducted into the IIHF Hall of Fame as part of their 2025 class, the third inductee from Denmark, after Jesper Damgaard and Jørgen Hviid.