By Dan Durkin—

Editor’s note: This is the fourth piece in a series previewing the 2015 Bears as the first training camp practice approaches July 30. You can find them in one spot here as they’re posted.

(CBS) Over the past two seasons, the Bears’ once-proud defense has sunk to the bottom of the league.

In 2013, the run defense was a doormat for opposing running backs. Last season, it was the pass defense that presented sieve-like resistance. The Bears ranked 30th in both passing yards allowed (4,230) and yards per attempt (8.1) and 31st in touchdowns allowed (34) and opponent passer rating (101.7).

Granted, a successful pass defense requires rush and coverage units to work in tandem, but the Bears ranked 14th in the league in sacks with 39. Obviously, those average numbers can be improved upon, but talent, communication and schematic issues plagued the defensive backfield.

Once Charles Tillman’s triceps gave way (for the second straight season) in the third quarter of the team’s second game against San Francisco, the game of attrition started at cornerback, and the lack of NFL-ready depth at the position was exposed over the course of the season.

Heading into 2015, the Bears are counting on Kyle Fuller to take a big step in his development, and two veterans from a group of three – Tim Jennings, Alan Ball and Tracy Porter – to lock down outside and slot cornerback roles on a defense that projects to be in nickel nearly 60 percent of the time.

Likely starters:
Right cornerback: Kyle Fuller (23, second year)
Left cornerback: Tim Jennings (31, 10th year)

Other competitors: Alan Ball (30, eighth year), Tracy Porter (28, eighth year), Sherrick McManis (27, sixth year), Demontre Hurst (24, second year), Terrance Mitchell (23, second year), Al Louis-Jean (21, second year), Qumain Black (23, rookie), Bryce Callahan (23, rookie) and Jacoby Glenn (21, rookie)

Key contributor: Kyle Fuller

The original plan for Fuller’s rookie season was to have him work at left cornerback in nickel situations, sliding Jennings into the slot corner role. This made sense, as decision-making – such as formation and alignment keys — from the slot has to be made in a split second, and the player must be a strong and willing run defender. These traits are/were a strong match for Jennings.

However, once Tillman’s injury struck, Fuller was forced into a full-time role at right cornerback. He responded immediately, with two late interceptions against the 49ers that helped the Bears complete an improbable comeback victory. But over the course of the season, his play went on a steep decline.

Certainly, hand and hip injuries played a role in Fuller’s decline, but in a no-mercy league, he became the mark for opposing passing games. His jam at the line wasn’t effective, and he was frequently caught flat-footed in his backpedal, leaving him stationary when receivers came out of their breaks.

The skills and instincts are there for Fuller; he just needs to be more technique sound in-route and more aware of route combinations to pattern match and break on the ball.

Finding cornerstone pieces on the Bears’ roster is challenging, yet coach John Fox and defensive coordinator Vic Fangio see nothing but success in Fuller’s future.

When asked at the NFLScouting Combine about which players Fox saw as being bright spots on defense, Fuller was the first name he mentioned.

“Fuller is a guy — my evaluation is more from what he did in college, where we had him in the draft process than it is completely on Bear tape — but I’d say a young guy like that, a core guy, he was a first-round pick,” Fox said. “I think he has the right kind of tools to be a guy that can lean into that leader spot as he goes.”

In Fangio’s scheme, Fuller is going to be asked to play more press-man coverage on an island. In a division featuring Calvin Johnson, Golden Tate, Jordy Nelson and Randall Cobb, he must develop into a true cover-corner in order for Fangio’s mixed-coverage schemes to succeed.

Potential breakthrough player: Alan Ball

Ball’s contract (one year, $3 million) certainly looks like starter money. It’s a three-wide receiver league on offense, and it’s a three-cornerback league on defense. Physically, Ball fits the long-armed profile that general manager Ryan Pace has targeted in cornerbacks. Schematically, he possesses the press-man skills Fangio seeks.

Ball spent the beginning of his career playing out of position as a slot cornerback and safety. It took Gus Bradley’s arrival in Jacksonville to get him into his best position (outside) and technique (press-man). In 2013, Ball was a reliable cover corner for the Jaguars. His 2014 season was cut short by a biceps injury, from which he’s fully recovered heading into this season.

Ball has some technique issues he can clean up, specifically being better with his jam technique at the line. But he’s a natural knee-bender who can sink and fluidly flip his hips to stay with a receiver, and his length is an asset on contested passes.

Ball has an opportunity to play the role Fuller was slated for in 2014, meaning he could play outside corner in nickel situations and allow the team to slide Jennings inside.

Final thoughts: Pace brought in veterans Ball and Tracy Porter to bolster what was and is a position with a lack of quality young depth. Players like Demontre Hurst and Al Louis-Jean got playing time last season, but if you isolate them on film, neither should be considered anything more than core special teams players.

Jennings parlayed 13 interceptions and two Pro Bowl appearances over the course of the 2012 and 2013 seasons into a four-year, $22.4-million contract with $11.8 million guaranteed. Last season, he didn’t register a single interception. Considering he’s the ninth-highest paid player on the 2015 Bears payroll ($5.25 million), he must return to his opportunistic self in a scheme that should generate some rushed throws over the middle.

Fangio will play Cover-2 shells, particularly against Aaron Rodgers, but he does prefer to leave his corners isolated, which helps him be more creative with how he uses his safeties in coverage (both as a blitz replacement and isolation) and against the run.

But the bottom line for this patchwork group is if Fuller doesn’t take the next step in his development and is unable to match an opponent’s top threat each week, the entire unit will be compromised.

Position grade: C.

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