By Dan Durkin–

Editor’s note: Last week, we broke down the future of the Bears’ key offensive players. You can read that here. Today, we focus on the defense.

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(CBS) As the NFL prepares to crown its conference champions and determine who will play for the right to hoist the Vince Lombardi trophy, there’s no question the league’s four best teams are still alive.

The Panthers, Cardinals, Broncos and Patriots have prominent quarterbacks, three of which are former first-overall selections, and the one who isn’t — Tom Brady — may be the greatest quarterback in the history of the league.

However, the offensive prowess these teams possess — Denver excluded — shouldn’t cast a shadow over what they are doing on the other side of the ball. This quartet of defenses is equally stout.

At the heart of each team’s defensive success is roster continuity, well-imagined and coordinated schemes and obviously difference-making talent.

Each defense ranked in the top nine in total yards allowed and rushing yards allowed per game, as well as in the top 10 in points allowed per game.

Each of New England’s projected 11 defensive starters were on the roster in 2014, of which the Patriots drafted eight. In Arizona and Denver, 10 of their starters were on the 2014 rosters, of which eight and seven were drafted by the team, respectively. In Carolina, seven starters were on the roster in 2014, but one defensive starter is rookie linebacker Shaq Thompson.

Applying that same analysis to the Bears illustrates how far their defense needs to go and grow together as they prepare for a radical overhaul this offseason.

The last time we saw the Bears line up, only five players on their defense were on the team in 2014 and just three were drafted by the team.

Over the course of the 2015 season, the Bears used six rookies as starters on defense. They combined for 41 starts, the second-highest total in the league. Such numbers are indicative of the developmental and transitional year 2015 was as the team sought to develop a defensive identity.

Despite playing with below-average talent at every level, defensive coordinator Vic Fangio helped this unit improve from the previous season. That’s not damning with faint praise, given how poor of a job predecessor Mel Tucker did on the job. The Bears improved in points allowed, total yards per game and passing yards per game.

Who’s part of the Bears’ defensive core moving forward? Who are the players who can be counted on to be reliable starters or, better yet, difference-makers? Let’s take a look at what the future holds.

Defensive tackle Eddie Goldman
Contract status: signed through the 2018 season

Goldman was a fringe first-round talent who the Bears were able to get in the second round. After struggling with his conditioning throughout the offseason program, he was thrust into a more prominent role after Jeremiah Ratliff’s bizarre episode that led to him being released from the team, and Goldman produced with the extra reps.

Goldman appeared in 15 games, starting 12, collecting six tackles for loss and four-and-a-half sacks, which ranked fourth overall for rookies and first among interior defensive linemen. He was stout and powerful at the point of attack and showed quickness converting from a run to pass key. He will anchor the interior of Fangio’s 3-4 for years to come.

Outside linebacker Pernell McPhee
Contract status: signed through the 2019 season

After the Bears made McPhee their top target in free agency last spring, the question became, would he be as or more productive with increased snaps? He answered that question with a resounding yes.

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Not only is McPhee a relentless pass rusher who can win with both speed and power, he’s as responsible against the run as any outside linebacker in the league. He finished the season with a team-high 25 quarterback pressures, 10 tackles for loss, six sacks, three passes broken up, an interception, a blocked field goal and a forced fumble.

Down the stretch of the season, McPhee was clearly hampered by a knee injury, which caused him to miss games. What he does in the offseason to remedy this remains to be seen. He proved to the coaches he’s an impact player. The team must now find a complementary outside linebacker to play opposite McPhee this offseason.

Safety Adrian Amos
Contract status: signed through the 2018 season

As if the speed of the carousel of players the Bears used at safety over the years didn’t make you nauseous, their play certainly did. By any measure, fifth-round rookie Adrian Amos solidified the free safety position for the Bears and became one of the biggest bright spots in an overall dim season.

Amos started all 16 games and played in 1,027 of their 1,030 defensive snaps, missing only three snaps against the Seahawks. He was the first rookie since Brian Urlacher to lead the team in tackles with 108 stops.

Amos was most effective early on as the team’s single-high free safety in Cover-1 and Cover-3 looks. He struggled when forced to play in the box and when isolated on receivers at strong safety in Antrel Rolle’s absence. For as many snaps as he played, his pass disruption numbers should be higher. He finished with only four passes broken up and no interceptions.

Fangio likes to use his safeties interchangeably, so where Amos ultimately ends up will depend on what the team is able to find in free agency and/or the draft. However, he’s proved to be a solid building block on defense.

Cornerback Tracy Porter
Contract status: unrestricted free agent

After tweaking a hamstring in training camp, there was a question whether Porter would leave Bourbonnais with a spot on the final 53. That same injury cost Porter the first two games of the season, but once he got an opportunity to play, he shined.

The first demonstration of the depth of trust the coaching staff had in Porter came against the Raiders. The Bears had him travel with rookie Amari Cooper, whom he held to just three receptions and 23  yards, shutting him out entirely in the second half of the game. At that point, it became evident that it was Porter, not 2014 first-round draft pick Kyle Fuller, whom the team viewed as its cover corner.

Porter went on to lead the team with 22 passes broken up, including four against Aaron Rodgers in the Bears’ Thanksgiving night win against the Packers in Green Bay. In the NFL, your secondary and pass rush must make plays to close out games, and Porter was often up to the challenge. While he probably projects best as a No. 2 cornerback, his play in 2015 should make him a priority to re-sign in free agency.

The Bears played 13 different players along their defensive line in 2015. Such numbers speak of a team seeking answers along their defensive front. Other than Goldman, the players who were used are primarily rotational pieces. The team needs two “5” techniques who can both set the edge against the run and also be able to win singles as offensive lines slide protection toward McPhee.

The biggest need is at inside linebacker. The scheme Fangio developed in San Francisco was predicated on elite play from his inside linebackers. The Bears cycled through four players — Shea McClellin, Christian Jones, Jonathan Anderson and John Timu — in various packages while seeking the right combination, which never was found. Rookies like Timu and Anderson gained valuable experience, which will benefit the depth of the team, but starters are needed at both positions.

In the secondary, the Bears played the season at least two players short. Their slot cornerback in nickel packages was a liability all season, particularly when Sherrick McManis manned that role. Rookie Bryce Callahan brought some stability to that position, playing with instinct and quick feet to pattern match and make plays on the ball, but he may be best suited as a dime cornerback.

Fuller’s inability to take a big step in his second season was a disappointment. It’s another first-round miss for the team, and upgrades at cornerback are needed to compete in a pass-heavy division.

Armed with the second-most free cap space and nine draft picks, the Bears will likely use the majority of their resources to rebuild their sagging defense. Fangio’s ability to keep the team in most games is a testament to his ability to game plan and prepare his team each week. However, this defense can’t take the next step without adding playmakers at every level.

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Dan Durkin covers the Bears for CBSChicago.com and is a frequent contributor to 670 The Score. Follow him on Twitter at @djdurkin.