There, he failed to make the team, and was sent on loans to Sparta, NAC Breda and Cambuur, emerging as top goalscorer of the Eerste Divisie with the latter.

[5] On the opening day of the 2013–14 Eerste Divisie season on 3 August 2013, Mühren scored all of Volendam's goals in a 3–2 away victory over Almere City.

[6] In a KNVB Cup match against Eredivisie heavyweights Ajax on 25 September 2013, Mühren netted a brace as Volendam fell to a 4–2 defeat in extra time.

[7] The game came just six days after the death of Robert's uncle Gerrie Mühren, who started his career with Volendam before going on to be a part of the great Ajax teams of the 1970s that won three consecutive European Cups.

[14] Mühren scored a number of important goals for AZ in the first half of the 2016–17 season and grew into a starter, but soon lost his place in the team due to the emergence of Wout Weghorst.

[15][16] After two-and-a-half years with AZ, some of which were spent with the reserve team, he moved to Belgian Jupiler Pro League side Zulte Waregem on 31 January 2017.

[17][18] Mühren scored two goals in 12 games for Zulte Waregem, but left on loan after serving out only six months of his four-and-a-half-year contract, after having disagreements with manager Francky Dury.

[32] He helped Volendam reach promotion to the Eredivisie in his first season back, scoring 29 goals and providing 12 assists in 37 league appearances.