Some potential replacements for Perry Fewell as defensive coordinator of the Giants:
Steve Spagnuolo: The former Giants defensive coordinator would be a popular choice with fans and the front office, but there likely will not be any players left on the 2015 roster who played under him during his 2007-08 tenure with the team. He hasn't had much success in high-profile roles since leaving New York -- three years as coach of the Rams and one as defensive coordinator of the Saints -- but he's in his second season with the Ravens now as a secondary coach and assistant head coach. That last title means the Ravens can block him from taking any position other than a head coaching opportunity if they want to. At the very least, the Giants will have to wait until the Ravens' playoff run ends before they ask for permission to speak with him.
Mike Smith: Tom Coughlin issued an "Ice Bucket Challenge" to the then-coach of the Falcons in the preseason. Will he issue a "Fix My Defense Challenge" to him now? Smith was fired from the Falcons at the end of the season and hasn't received many nibbles from teams with head coaching openings. Smith was an assistant on the Ravens' dominant defensive teams from 1999-2002 and defensive coordinator for the Jaguars before joining the Falcons in 2008. He and Coughlin have a close relationship.
Jim Schwartz: The Bills defensive coordinator is still under contract with the team and is even a candidate for its head coach opening. If he doesn't get that job, though, there's a good chance whoever does will want to bring in their own guy and Schwartz would be free to pursue other job opportunities. And there will be a few. Other teams are interested in him as a coordinator as well, including Washington. Schwartz also has head coaching experience with the Lions.
Vic Fangio: Another current defensive coordinator who, like Schwartz, is a candidate for his team's head coach opening. It seems unlikely Fangio will want to remain with the 49ers if he doesn't get the top job, and while he could be a candidate for other head coach positions he'll also be a hot defensive coordinator prospect. Fangio is known as a brutally honest straight shooter. He also built the 49ers' defense around their linebackers in a 3-4 system, which is not something the Giants have done under Coughlin, although the prospect of developing Devon Kennard could be enticing.
Pepper Johnson: If the Giants want to go with a first-time coordinator the way they did with Ben McAdoo on offense a year ago, Johnson could be a strong candidate. He won two Super Bowls with the Giants as a player and spent 14 years on the Patriots' coaching staff. He's currently the defensive line coach for the Bills, which means Buffalo cannot block him from interviewing for a coordinator's position.
Other names to consider: Dennis Allen, Mike Nolan, Al Holcomb, Darren Perry.