"The Pats need a pass rusher in the worst way ...."
I read this cliche every year. But the BB's defenses are far less "attack-based" than pretty much every other defense in the league, even than the other 3-4 schemes. And, when the defense is in attack mode, the primary rusher may be a different guy each time and might come from any one of a number of positions. So, "a pass rusher" is not something that BB seems ever to have actually had on his Pats teams or looked for in the draft, especially in the first round. Overall, your description of Mercilus - "relentless, good inside power, sets the edge well (from the down DE spot)" seems to describe Andre Carter, who doesn't seem to do any of these things nearly as well standing up at OLB. Unless BB is well-assured that Mercilus CAN be successful standing up (the "one-year wonder" thing is a significant detraction here), then I don't see BB spending the #27 on him. He can re-sign Carter or another FA (Trevor Scott, perhaps) or use a later pick for the role that Mercilus seems, at this point, suitable for in BB's scheme.
A bigger need is probably that standup OLB. Nick Perry - generally stronger and more athletic than Mercilus, and with a longer resume, and appearing to have mostly correctable technique issues - might be the better choice, though I have my doubts that BB would be sold on Perry, either.
If the Pats are seriously concerned that the injury to Ras-I will be a long-term thing (and, since the Pats weren't at all active in the FA CB market, I doubt that this is the case), then maybe they'd take yet another CB in the 1st or 2nd (though there seems to be some positional depth in this draft class, so, probably not). Regardless, I have serious doubts that BB would invest the #31 in so obvious a problem child as Jenkins, and especially if he reminds BB of a freelancer like Samuel - who would absolutely make this very young secondary even worse. If BB spends a high pick on a DB in this draft, it seems much more likely to be a very smart - and "obedient" - coverage safety.
