CHARLOTTE, N.C. -- The Carolina Panthers' 2016 draft class was mostly about replacing Pro Bowl cornerback Josh Norman and upgrading the defense in general.

It’s still a work in progress.

The Panthers used three of their five picks to select cornerbacks between the second and fifth rounds. The hope was one would fill the big shoes of Norman, who signed with Washington after Carolina rescinded the franchise tag.

Second-round pick James Bradberry has played well at times, but a toe injury forced him to miss three starts. Third-round pick Daryl Worley has steadily gotten better, picking up his first career interception on Sunday at San Diego. Fifth-round pick Zack Sanchez went on injured reserve in November.

Carolina’s pass defense, ranked 30th in the NFL, was a major reason for a 1-5 start that all but ended Carolina’s chances to make a fourth straight playoff appearance.

But it's gotten better, working more in tandem with the front seven over the past seven games like it did last season. The Panthers, according to ESPN Stats and Information, have gone from allowing a 54.3 completion percentage in their first six games with eight touchdowns to only three interceptions on passes 15-plus yards down field to 35.3 percent with two touchdowns to six interceptions in the last seven games.

Grade: C-minus

Best rookie: Second-round pick Bradberry hasn’t made the impact plays that Norman did. He has only one interception and no forced fumbles. But he did show promise before the toe injury of being able to cover the opponent’s best receiver and he's been steady of late.

Most improved rookie: Third-round pick Worley. He simply wasn’t ready to start when the season began and that was evident when he made his first against Atlanta. But he has settled down as the season has progressed and is fourth on the team in tackles with 68, including a sack.

Most disappointing rookie: First-round pick Vernon Butler. You expect your first pick to make a major contribution, but the defensive tackle out of Louisiana Tech hasn’t, primarily because of a high ankle sprain suffered early in the season.

The jury is still out on ...: Vernon Butler. As mentioned above, the ankle injury slowed Butler’s progress and kept him from being an impact player. Coach Ron Rivera said the ankle won’t fully heal until after the season. But Butler did show flashes early, getting a sack in the opener at Denver. Way too early to call him a bust.

Undrafted rookie check in: Not much to check in on here. Linebacker Jeremy Cash and wide receiver Keyarris Garrett were considered two of the best players still on the board after the draft. Cash was showing promise on special teams early, but an early-season injury set him back. Garrett has spent the season on the practice squad.