The ascension of the San Francisco 49ers’ defense from the NFL's worst unit by PFF grade in 2018 to one of the best defenses in the league this season — regardless of the metric that you look at — is arguably the biggest turnaround by any group in the NFL this year. The massive improvements to their pass defense were the driver of that rise, and they can be attributed to two areas: coverage and pass rush.
It’s hard to overstate how important their back-end improvements have been. Richard Sherman has come back from an Achilles injury that ended his 2017 season to be named PFF’s top coverage defender in the NFL in 2019.