Top offseason needs: AFC North

The Baltimore Ravens could use some defensive line depth behind Haloti Ngata. Mitch Stringer/USA TODAY Sports

Welcome to the 2013 version of Insider's offseason staple series: Plugging the Holes. This marks the second of an eight-part examination in which Football Outsiders identifies the biggest needs for every NFL team, division by division, going into the 2013 offseason.

The series continues Tuesday with Outsiders' look at the AFC North.

Baltimore Ravens: Defensive line

The Ravens are champions once again, but this year's ring was primarily won thanks to the surprise source of most of Baltimore's regular-season success: the offense. The Ravens improved a bit on defense in the postseason, but during the regular season they had fallen precipitously to 21st in Football Outsiders' DVOA ratings after ranking first in 2011. (DVOA is Football Outsiders' defense-adjusted value over average metric, explained here.)

The Ravens also declined against the pass, going from first to 13th. But the real decline came against the run, where Baltimore fell from seventh in DVOA (giving up 3.5 yards per carry) to 26th (4.0 yards per carry against a fairly easy schedule of opposing running backs). The biggest problem came up the middle.

If you look at Football Outsiders' adjusted line yards numbers, which cut runs at specific distances to gauge the run-stopping ability of the front seven, the Ravens finished 23rd, 29th and 29th in runs listed as left tackle, middle/guard and right tackle, respectively.

Haloti Ngata was selected for his fourth Pro Bowl, but lingering shoulder and knee injuries dampened his performance much of the season. Ngata's MCL was reinjured in the Super Bowl, and even if he returns to his old pre-2012 self in 2013, the problem is figuring out who to put next to him.