A disposable reservoir unit configured for releasable attachment to a reusable drive unit of an injection device includes a reservoir holder for holding a reservoir, two radially extending and oppositely arranged protrusions for releasably attaching the reservoir unit to the drive unit, and a needle cover sleeve guided by guiding means and movable along the longitudinal axis relative to the reservoir holder between a covering position and a retracted position. In a plane perpendicular to the longitudinal axis the needle cover sleeve has a first outer dimension along a first axis and a second outer dimension along a second axis perpendicular to the first axis, the first outer dimension is larger than the second outer dimension, and the protrusions are arranged on the first axis.