San Francisco 49ers: Report-card grades & analysis from Weeks 1 through 4

Outside of Richard Sherman, who has been effectively shutting down the left side of San Francisco’s defensive backfield, the 49ers secondary has been one of the primary weaknesses over the first four games of 2018.

Granted, injuries have played a big role. In addition to Sherman and Jaquiski Tartt, No. 2 cornerback Ahkello Witherspoon has been hobbled by an ankle injury. Free safety Adrian Colbert is also dealing with a knee injury.

That said, the Niners secondary has not looked good over the course of the season. This unit was carved up by the Detroit Lions in the second half of Week 2 and had few first-half answers against the Chiefs the following week.

Speaking of maturation, Pro Football Focus ranked Witherspoon the No. 103 cornerback out of 104 players graded entering Week 4. And while he played better in Week 4 in Los Angeles, it’s pretty clear the former Colorado defensive back is having a bit of a sophomore slump.

What’s Good

Sherman is becoming the 49ers’ first shutdown corner in some time, allowing just six targets and one catch on a total of 123 coverage snaps thus far. He’s been as good as possibly could have been hoped for after signing this offseason.

Reserve defensive back Antone Exum made some nice plays against the Chargers too, including a pick-six touchdown — the Niners’ first interception of the year.

What Needs Improvement

The 49ers have now allowed 1,105 pass yards over four weeks — and average of 276.3 per game — which was skewed down by giving up only 242 in Week 4.

Witherspoon’s struggles have been noted. And the longer the season draws on, it might be time to see if San Francisco has anything of promise with its younger defensive backs, namely Tarvarius Moore and Reed, who seems to be an upgrade over Colbert right now.

Of course, this unit would be greatly assisted by a pass rush.