In today's NFL, the spread offenses from college have become a major part of the game. Then Tom Brady , Kurt Warner , and Peyton Manning bursted onto the scene, winning championships and setting records throwing with 3+ WR formations that took advantage of teams that either put a linebacker or a backup CB on the 3rd WR back in the early 2000s. The NFL is a copycat league, and most teams adapted that to their offenses. To counter that, most teams now will sub in a defensive back for either a linebacker (4-3) or nose tackle (3-4).
In either case, the Patriots are playing with two traditional linebackers in the formation. In 2014, the Patriots utilized Dont'a Hightower and Jamie Collins in that role. Both players excelled in that role, and will likely see more of the same in 2015. However, a complicating factor is Jerod Mayo and his very large cap hit ($10.3M). Mayo has only played in 12 games the last two seasons, which put Collins and Hightower on the spot in both 2013 and 2014. The Patriots may end up restructuring the deal to lower his 2015 hit, but it likely increases his cap hit in 2016. In that scenario, the Patriots will likely end up cutting him because he's not worth a $11M+ cap hit now since they have two young options that they'll need to extend for the future.
With Mayo likely a 2016 cap casualty due to both the scheme and the need to pay younger players, the Patriots could end up turning to a linebacker in the draft. I'm not sure what they'll do for the 4-3 base, because Hightower and Collins would be OLBs and the team would be looking for a MLB. Internally, they could set up a LB trio of Collins-Hightower-Fleming. Darius Fleming is a core special teams player, but also has the size that Belichick likes for his linebackers. However, the good news is that both players are very flexible enough to play any LB position in any scheme, so the Patriots could draft an OLB and slide Hightower inside for base D. If the Patriots move into a 2-4-5 (or 4-2-5 depending on how you view Chandler Jones and Rob Ninkovich in a 2-Pt stance), then they would likely have to sub that LB off the field. With a part-time player, ideally he can also contribute on special teams.
Some names to consider in the draft are Ben Heeney, Mike Hull, Curtis Grant, and Zach Vigil. All of whom are likely taken on the 3rd day of the draft. In addition to the potential picks in the draft, the Patriots have Fleming, Cameron Gordon, and James Morris as potential depth options in camp for the LB spot. With Hightower and Mayo rehabbing from injuries, someone is going to have to take the bull by the horns and step up in camp. Gordon and Morris were IR'd last season, so between them and a potential late round or free agent rookie, there is one spot open in camp for a reserve LB role. We'll see what the Patriots do at the position in the draft. (Since I published this post, they'll probably draft one early just to spite me)