It's not easy for an NFL defense to go from subpar to above average in one year, but the Cardinals believe they have the ingredients to pull it off.

Even though wideout DeAndre Hopkins was the marquee addition this offseason, General Manager Steve Keim has poured the majority of his resources into the defense.

In the past few weeks, multiple players have spoken about the changes, and the optimism is palpable, for several reasons:

• The Cardinals finished 2019 last in total defense, 28th in points allowed and in the bottom quadrant of most other categories. However, the final four games were a marked improvement, as they allowed 329 yards and 22.8 points per game over the final month.

Some might say it was random variance that the team looked good down the stretch, but cornerback Patrick Peterson saw a group meshing. The Cardinals were learning a new system last season under defensive coordinator Vance Joseph and dealt with myriad personnel changes due to suspension, injuries and cuts.

Peterson said the communication and chemistry peaked at the end of the season, which was a critical sign for the group's 2020 hopes.

"Does that mean anything?" Peterson said. "You're damn right it means something."

• The Cardinals' front seven had serious holes last year. While outside linebacker Chandler Jones, inside linebacker Jordan Hicks and nose tackle Corey Peters were productive, they were flanked by several players better-suited for reserve roles.

Keim addressed that in free agency by adding defensive tackle Jordan Phillips, edge rusher Devon Kennard and inside linebacker De'Vondre Campbell, a trio of proven NFL starters. Campbell believes the free agent moves will complement that late-season improvement.

"I think I can be a huge piece of that (progression), and everybody else that's new in the organization," Campbell said.

• The Cardinals added four more defensive pieces in the draft, headlined by rangy linebacker Isaiah Simmons with the No. 8 overall pick. Peterson believes Simmons can be Budda Baker's sidekick as a versatile piece in the middle of the field.

Jones said he would have been fine if the Cardinals went offense at No. 8, but once Simmons dropped, he was happy Keim pounced.

 "Watching a guy like that, you can't not draft him," Jones said.

The Cardinals had serious issues covering tight ends last season, and Simmons' skillset seems a perfect antidote.

• The Cardinals had a projected starting secondary of Peterson, Baker, Robert Alford and D.J. Swearinger last offseason. But Peterson was suspended for six games, Alford suffered a season-ending broken leg in training camp and Swearinger was released early after struggles in coverage.

Predictably, the young replacements took their lumps, as the Cardinals finished 27th in the NFL by allowing 7.0 net yards per pass attempt.

If the Cardinals can turn things around, much of it will be due to improvement from the defensive backs. Peterson and Alford are ready to contribute in Week 1, while Jalen Thompson formed a nice tandem with Baker down the stretch. If Byron Murphy can play well in the slot, the secondary could be much-improved.

"With me and Pat being back, and Budda, I feel like the sky is the limit for us in the back end, and for us as a whole on defense," Alford said.

The Cardinals must make major strides in order to leave 2019's struggles in the rearview mirror, but between the roster upgrades and the cohesion that developed late in the year, the group believes it is well on its way to a turnaround.

"It's not like we're going to go out there fishing for plays or fishing for certain things to work," Peterson said. "(Joseph) knows what works. He knows what certain players like. He's going to run with that, and I believe if we can do that for 17 weeks -- keep teams out of the end zone and hold them to that 15-point mark, which is our goal every Sunday -- we can be a top-tier defense."