This is the fourth part of an eight-part series in which Football Outsiders looks at the biggest post-draft needs for every NFL team. This week will consist of the AFC teams, by division, while next week will focus on the NFC.
Today, Outsiders looks at the AFC North.

Baltimore Ravens: Offensive line
You'd think the Ravens would have learned their lesson. Just prior to last season, the potential absence of center Matt Birk and left guard Ben Grubbs meant that most of the offensive line consisted of players without positions. Michael Oher was a left tackle, a right tackle, or a right guard. Marshal Yanda was being considered at both tackle and guard on the right side. Andre Gurode could replace either Birk or Grubbs, but not both. And then, miraculously, everything fell into place with the resurrection of Bryant McKinnie's career.
Yet here we are entering 2012, and it's deja vu all over again.
This time, Grubbs is gone for good, having left for New Orleans in free agency, Birk has chosen to forego retirement but is still on the wrong side of 35 years old, McKinnie finds himself being monitored by literal "weight watchers," and Gurode hasn't been re-signed. Naturally, Baltimore's addressed this situation by moving backup tackle Jah Reid to guard, drafting Iowa State tackle Kelechi Osemele, who the Ravens envision as a guard prospect, and selecting Delaware guard Gino Gradkowski to one day replace Birk at center. Follow all that? Yeah, me neither.