A part of a device for injecting a liquid product having an end sleeve and a syringe support which is able to move with respect to the end sleeve. The syringe support is designed to bear an injection syringe fitted with a removable protective cap. A deformable ring is kinematically connected to the syringe support. This ring can be deformed elastically between a configuration which is designed to axially immobilize the injection syringe in the syringe support, and a configuration designed to allow the protective cap to pass through the deformable ring.