The issue is most likely that, while he has experience in 3 and 2 point stances, he lined up primarily as an edge rusher in college. To do that in the NFL, he'd have to be either a 3-4 OLB or a 4-3 DE, and as a 4-3 DE he's likely to get physically dominated on running plays by NFL tackles. He'll need to be a good amount stronger to be able to play on the defensive line, which means that he's probably going to be a 4-3 OLB. On the face of it, it becomes hard to utilize his pass-rushing talents from this position, but who knows what Belichick has in mind for him. Maybe it's as simple as just using him on obvious passing downs, and not worry about teams running at him. Maybe the Pats will rush him on some downs while dropping one of the DEs into coverage to confuse the offensive line.
Hard to say, but I wouldn't be shocked if this signals a longer-term intention to shift back to a 3-4 base, with Collins and Jones ideally being OLBs, Mayo and Hightower at ILB, and Wilfork, Armstead, and a yet-to-be-signed third on the defensive line. If this is the case, I think it'll manifest itself over the next 1-2 years in the form of Belichick stockpiling a lot more DE-DT hybrid types; guys who can two-gap.