Eight in the Box: Must-keep free agents

Welcome to "Eight in the Box," a new NFL Nation feature that will appear each Friday during the offseason. This week’s topic: Which free agent is essential for each team to keep from its 2012 roster?

Chicago Bears: In his two years as a starter, Henry Melton has 13 sacks -- more than all but one NFL defensive tackle over that stretch. Some argue his skills only fit certain schemes, but if that's the case, the Bears' new coaching staff should make sure it runs one that allows Melton to continue rushing the passer.

Detroit Lions: Defensive end Cliff Avril has his detractors, but there are plenty of teams that would love to have a player who has collected 29 sacks in his past three seasons. Safety Louis Delmas is important as well, but he has trouble staying healthy while Avril has started 40 consecutive games, including playoffs.

Green Bay Packers: The team can probably absorb the expected departure of receiver Greg Jennings, but there should be no debate about the value of keeping cornerback Sam Shields, a restricted free agent. Shields' starting-caliber play late last season means the Packers have no choice but to issue him a high enough RFA tender to prevent him from signing elsewhere.

Minnesota Vikings: The Vikings were one of four teams to use the same starting offensive line all season, and right tackle Phil Loadholt was a big part of their success. He won't command elite money and wants to return, so a deal shouldn't be difficult.