When you get right down to it, the Bills only have five legitimate, NFL-caliber defensive linemen on their roster. Kyle Williams is awesome. Dwan Edwards has been solid before (but wasn't often in 2010), and Spencer Johnson has his moments as a backup. Torell Troup and Alex Carrington are second-year guys looking to become contributors.
That's it. Even if the team were strictly a 3-4 defense (they're not), that's scary thin depth.
Size and versatility is needed, in the form of at least one, but perhaps two young bodies. That's particularly true if the team plans to continue its frequent use of a four-man line. Williams, Edwards and Carrington can be penciled in as starters for now, but clearly, depth is a major, major issue up front.