Rothmann started on the bench, but replaced Mads Valentin in the 55th minute in a 0-1 defeat against Vendsyssel FF in the Danish Cup.
On 9 January 2019, Rothmann signed his first professional contract and would be permanently promoted to the first team squad ahead of the 2019-20 season.
[2] On 12 May 2021 Nordsjælland confirmed, that Rothmann had signed a new deal until June 2022 and immediately would be loaned out to Norwegian Eliteserien club Tromsø IL for the rest of 2021.
On 29 August 2021, one year before his contract with Nordsjælland expired, Rothmann was once again sent out on loan, this time to Danish 1st Division club HB Køge for the rest of the season.
[8] In early January 2025, newly promoted Allsvenskan club Degerfors IF confirmed that Rothmann would be on trial for the next two weeks.