The D/L will have primary on this. Even in 7-8 man with a U in the traditional position the H/L play a key role in this. The L holds the LOS until the pass crosses so they can be a big help. The U only covers the LOS secondarily and since most IDP fouls involve run reads, he's probably still 5-7 yards beyond the LOS at the time of the pass when one of his keys releases at the snap.