In the following 2016–17 season, Geertsen was assigned to continue with the Rampage, on October 1, 2016.[1] Used primarily in a defensive role adding physicality, Geertsen appeared in 36 games with the Rampage for 4 assists. He split the season again between the AHL and ECHL joining secondary affiliate, the Colorado Eagles in time for the postseason. Geersten remained with the club through the postseason, establishing a regular spot on the defence in recording 1 goal and 4 points in 19 games to help the Eagles capture their first Kelly Cup.[2]
As an impending restricted free agent following the conclusion of his entry-level contract, Geertsen's tenure with the Avalanche ended after he was not tendered a qualifying offer on June 25, 2019.[3] As a free agent over the summer, Geertsen was invited to attend the New York Rangers 2019 training camp. After a successful camp, Geertsen was among the Rangers second round of cuts and was signed to a one-year AHL contract with affiliate, the Hartford Wolf Pack, on September 29, 2019.[4][5]
In the 2019–20 season, Geertsen established a role within the Wolf Pack's blueline as a physical stay-at-home defenceman, appearing in 60 regular season games and leading the club with 109 penalty minutes, before the season was cancelled due to the COVID-19 pandemic. On April 23, 2020, Geertsen was signed to remain within the Wolf Pack organization, agreeing to a one-year contract extension.[6]
Entering the pandemic-delayed 2020–21 season, Geersten registered one assist through four games before he was signed to a two-year, two-way contract with the NHL affiliate, the New York Rangers, on March 4, 2021.[7]
On October 3, 2021, Geertsen was claimed off waivers by the New Jersey Devils.[8] He made his NHL debut on October 19, 2021, in a game against the Seattle Kraken.[9] He would play 25 games for the Devils that season, and on July 13, 2022, he signed a one-year contract extension at league minimum salary to stay with New Jersey.[10]
Geertsen spent the 2022–23 season with the Devils' AHL affiliate, the Utica Comets, where he set AHL career highs of 4 goals and 136 penalty minutes.
As a free agent from the Devils, Geertsen was signed to a two-year, two-way contract with the Vegas Golden Knights on July 1, 2023.[11]
On November 22, 2024, ahead of the Silver Knights' annual Hockey Fights Cancer night, Geertsen released a statement announcing he was diagnosed with non-Hodgkin lymphoma, a form of blood cancer, in November 2023, adding that it was the cause of a one-month absence during the 2023–24 season. After undergoing treatment through the summer of 2024, Geertsen's cancer went into remission before the 2024–25 season.[12][13]