[2] Koubek made his league debut for Hradec Králové in a 2–1 win against Mladá Boleslav on 30 April 2011.

[3] After Hradec Králové were relegated at the end of the 2014–15 Czech First League, Koubek joined Sparta Prague on a four-year contract, immediately going out on loan to Slovan Liberec for the 2015–16 season.

[5] He mainly started on the substitute bench during the 2017–18 Czech First League, but was later given preference in goal over his competitor, Slovak goalkeeper Martin Dúbravka.

[10] On 11 January 2025, Koubek signed a contract with Czech First League club Slovan Liberec until June 2027.

[11] In October 2015, Koubek was called up by coach Pavel Vrba to the Czech senior team.