For all the talk about MVPs and offensive standouts, there haven't been a lot of defensive standouts. Robert Quinn (a pass rushing beast), Muhammad Wilkerson (second to only JJ Watt at 3-4 DE), and JJ Watt (leads the NFL in TFLs) are playing great but are on bad football teams and Quinn's sack numbers aren't that eye popping. At least not enough to overcome the team he is on. Justin Houston was playing very nice defense too but he's currently injured.
Here are, IMO, the main candidates in no order.
S, Earl Thomas, Seattle Seahawks - With all due respect to Richard Sherman, Earl Thomas is the top player in the Legion of Boom. He has wicked range for a safety, plays the run exceptionally well, and covers ground in the pass game better than any other safety in the NFL. However, he is a DB and it takes a lot for that position to win this award. Plus he doesn't get much pub, either.
LB, Luke Kuechly, Carolina Panthers - Gets a bunch of publicity having already won DROY, 1 sack, 3 interceptions, 6 PDs, 8 TFLs, and is the star player on the best defense in the NFL.
OLB, Robert Mathis, Indianapolis Colts - Currently leads the NFL in sacks and has gotten a lot of pub recently. However, the Colts defense simply isn't all that good and nothing feels particularly special about his season, either.
OLB, Justin Houston, Kansas City Chiefs - With his injury, I doubt it. However, I think he was the front runner beforehand. Amazing pass rusher.