[8] During the winter break of the 2016–17 season, Arp was included with the Hamburger SV first team under head coach Markus Gisdol.

[12][14] Nevertheless, after scoring seven goals in three games in the Under 19 Bundesliga, manager Markus Gisdol called him up for the matchday squad for the first time for the Nordderby against rivals Werder Bremen on 30 September 2017.

[19][20] On the following matchday, Arp was in the starting line-up for the first time in the 3–1 win over VfB Stuttgart and scored the final goal of the match.

[26] Arp made his first appearance of the season for the first team in the first round of the DFB-Pokal in a 5–3 victory against fifth-tier club TuS Erndtebrück, where he contributed with a goal.

After Hannes Wolf took over the reins at the end of October 2018, Arp continued mostly as a substitute, and appeared as a left winger, while Lasogga and Hwang Hee-chan were given preference at the striker position.

[2] At the beginning of the season, now 19-year-old Arp was mostly not included matchday squad, as head coach Niko Kovač preferred second-team strikers Kwasi Okyere Wriedt and Joshua Zirkzee as backups to undisputed starter Robert Lewandowski.

[38] During his absence, the one year younger striker Zirkzee was promoted to the professional team by Flick scored two goals in two substitutions.

[39] In early February, he was removed from the UEFA Champions League squad for the knockout phase for winter transfer window newcomer Álvaro Odriozola.

[40] After the winter break, Arp was behind Wriedt, Zirkzee and Leon Dajaku on the depth chart for the reserves under head coach Sebastian Hoeneß.

After the resumption of play, Arp made a few more appearances for the reserves before injuring himself as he scored his third season goal for the team, once again sidelining him for some games.

[26] After a disappointing first season in which he failed to make a single Bundesliga appearance, Arp was made a permanent member of the club's secondary team in order to receive more playing time.

[43] For the reserves, Arp was initially a regular starter under new head coach Holger Seitz, but had to settle for a role as substitute behind Lenn Jastremski at times.

After the managerial duo of Danny Schwarz and Martín Demichelis had taken over the relegation-threatened reserves for the last 8 games of the season, Armindo Sieb was the preferred starter in attack.

He scored in the first round of the DFB-Pokal against the Regionalliga club SC Weiche Flensburg 08 and in the 3–0 victory on matchday five against Erzgebirge Aue.

[50] While his Austrian rival scored 6 goals before the winter break and became the team's top goalscorer, Arp was only used as a substitute and remained goalless.

[52] Arp was part of the German U17 team in May 2017, scoring seven goals in the 2017 UEFA European Under-17 Championship to finish second on the top goalscorers list behind Amine Gouiri.