The invention relates to a pump unit (2), replaceably attachable to a reusable backend (3) of an injection arrangement (1) for delivering a liquid medicament, the pump unit (2) comprising a medicament inlet (2.1), a medicament outlet (2.2) and a peristaltic pump for delivering the liquid medicament from the inlet (2.1) to the outlet (2.2), the peristaltic pump comprising a pump rotor (2.3) and a pump hose (2.4), the pump hose (2.4) partially arranged around the pump rotor (2.3), the pump rotor (2.3) having protrusions (2.5) for engaging the pump hose (2.4), wherein a fixing side (2.6) of the pump unit (2) facing a reusable backend (3) when attached has a recess (2.7) in the shape of a circular arc for allowing a correspondingly shaped stop (3.1) protruding from the reusable backend (3) to enter into the pump unit (2) so as to support the pump hose (2.4) from an outer side opposite the pump rotor (2.3) thus allowing the protrusions (2.5) to locally squeeze the pump hose (2.4) against the stop (3.1) when attached to the reusable backend (3).