Odds are excellent the Steelers will replace Dick LeBeau with their linebackers coach Keith Butler. That said, such an important position hire shouldn't be thoroughly investigated. While it's fair to point out LeBeau officially resigned, meaning he left on his terms, the decision was mutual, according to reports.
It wouldn't be a bad idea for the Steelers to kick over some rocks and look into external candidates, if for no other reason than for due diligence. Here's a list of some external coaches who may make good coordinators.
A resume rivaled by few others in terms of experience and defensive success, Phillips most recently put together an excellent string of defensive teams in Houston. He oversaw the initial development of All World defensive end J.J. Watt, and took a Texans defense at the bottom of the league in most defensive categories to the top from 2010 to 2011. A 3-4 guru, Phillips could be seen as a plug-n-play kind of coordinator; the personnel is already established, and he could work with Steelers coach Mike Tomlin to move this group in a different direction without having to worry about moving players to new positions.
Most likely, Seto is in line to take over the defensive coordinator position for the Seattle Seahawks, where he currently serves as the team's defensive passing game coordinator - likely a fancy name for a coach a team can't make a coordinator but not wanting him to leave for an outside job. Few can argue with the success of the Seahawks' defense as a whole, especially against the pass. While it's likely current Seahawks defensive coordinator Dan Quinn gets a head coaching gig after this season (the Seahawks advanced to the NFC Championship game with a win over Carolina) and Seto appears to be his likely replacement, a fresh approach to the Steelers' defense would be a good thing.
As of Sunday morning, it appears Ryan will be the next head coach of the Buffalo Bills.
This is the name thrown about all over social media and the Internet soon after announcement of LeBeau's resignation broke. Probably the least likely candidate, Ryan was fired by the Jets at the end of the 2014 season, and without knowing the specifics of his contract, terminated coaches usually continue getting paid their previous salary or take a lump sum buyout that's contingent the coach does not coach in the NFL for a certain amount of years. It would seem most likely Ryan could collect a nice salary from a TV network as well as from the Jets to not coach in 2015, but if he would coach, it would seem more likely he'd take a head coaching gig or none at all. Again, though, for the sake of due diligence, his name has to be brought into the conversation, even if nothing about him appears to be a good fit.
The former Tampa Bay Buccaneers head coach and longtime defensive assistant, Morris coached with Mike Tomlin under Tony Dungy in Tampa Bay in their early years in the NFL. Morris, currently the defensive backs coach with the Redskins, received permission last week to interview for the Giants' defensive coordinator vacancy, strongly suggesting the Redskins wouldn't stop him from a promotion. Coaches often hire coaches with whom they've previously worked, and Tomlin's connection with Morris shouldn't be overlooked. He's from the same Tampa 2 school Tomlin is, and his hire would suggest the team is about to switch its defensive alignment.
The former Giants great defensive lineman turned coach spent the bulk of his coaching career with the Patriots, helping mold the talented and versatile defensive front the Patriots always have. He was the linebackers coach as well as serving a stint as the defensive line coach in New England from 2000-13. He left in 2014 to be the defensive line coach in Buffalo, where he is now. He also has interviewed with the Giants for their defensive coordinator position.