  We suggest the measurement of Drell-Yan (DY) lepton pairs in p-Pb collisions
at the LHC (sqrt(s)=5.02 TeV) in order to disentangle the relative
contributions of shadowing and coherent energy loss in quarkonium production
off nuclei. The nuclear modification of low mass DY production is computed at
NLO using various sets of nuclear parton densities. It is then observed that
shadowing effects strongly cancel out in the J/psi over DY suppression ratio
R_{pA}^{J/psi}(y) / R_{pA}^{DY}(y), unlike the effect of coherent energy loss.
Such a measurement could be performed at forward rapidity by the ALICE and LHCb
collaborations at the LHC.
