We study the collisional energy loss mechanism for particles produced off mass shell in a finite size QCD medium. The off-mass-shell effects introduced consider particles produced in wave packets instead of plane waves and a length scale associated with an in-medium particle lifetime. We show that these effects reduce the energy loss as compared to the case when the particles are described as freely propagating from the source. The reduction in energy loss is stronger as the scale becomes of the order of or smaller than the medium size. We discuss possible consequences of the result on the description of the energy loss process in the parton recombination scenario.