Perrish Cox has turned into a solid cornerback, so what happens when Tramaine Brock returns?

The San Francisco 49ers defense has had its share of question marks, with the pass rush and the secondary both coming under criticism. However, one bright spot on defense has been cornerback Perrish Cox.

The 49ers inserted Perrish Cox into the starting lineup during the preseason while Tramaine Brock recovered from a sprained ankle. Brock returned for Week 1, but then injured his toe. Cox replaced him again, and has remained in the starting lineup while Brock gets healthy.

Cox first joined the 49ers as a free agent in 2012. For most of 2012 and 2013, he was a special teams contributor who was also the dime back. He was released midway through 2013 when Eric Wright was brought off the NFI list. When Carlos Rogers subsequently injured his ribs, Cox returned to play nickel back in the playoffs.

This year, Jimmie Ward is playing nickel back, and Perrish Cox is playing outside, opposite Chris Culliver. In three games of action, Cox ranks No. 5 on Pro Football Focus among all cornerbacks. Even if you choose to ignore such rankings, if you have watched Perrish Cox this season, he has been a bit of a revelation.

I took a look back at his snaps against the Arizona Cardinals. There were a couple plays I remembered specifically, but I wanted to take a look at all of his snaps, just to get an idea of what he brought to the table.

What I found most interesting was that Drew Stanton did not challenge him all that frequently. Drew Stanton did a good job spreading the ball around and looking all across the field.

PFF had Cox targeted five times, giving up two receptions for 17 yards. One of those receptions was a quick dump-off to Andre Ellington that gained ten yards. The Cardinals found a hole in the zone the 49ers seemed to be using, so I am not overly concerned about that for Cox.

Cox spent the entire time covering Stanton's right side of the field. He was frequently on Larry Fitzgerald, including the play below. I wanted to point to Cox's closing speed with this one. The first picture shows Drew Stanton (circled) starting to pass the ball. Fitzgerald has just started to cut back, and Cox is about five or so yards away from him.

This next picture shows the pass break-up from the same All-22 angle. Cox quickly closed and got a hand in on the pass to break it up. The pass itself should have probably been a little further to Fitzgerald's left, but in any case, Cox got there to break it up. It wasn't a spectacular play, but it got the job done.

Cox had one particularly spectacular play that everyone probably remembers. I turned into a GIF just so we could see how he hung with Michael Floyd and made a great play on the ball.

Cox was in coverage on Floyd. The Cardinals receiver got behind Cox, but he did not gain sufficient separation. Stanton dropped the ball in and Cox almost snagged the diving interception. If Stanton adds more touch on that and drops it a yard further, maybe Floyd gets it, maybe not. Either way, Cox did a great job keeping an eye on the ball and getting the jump at just the right moment.

I'm not entirely sure what changed from the last couple years to this year. Maybe just getting consistent playing time has allowed Cox to put it together. He was a talented cornerback coming out of college, but was inconsistent in Denver. Maybe the light went on a little bit later.

The big question now is what happens when Tramaine Brock returns from his toe injury? It might be another week or two (or more), but he should be back at some point. When that happens, where does Cox go? Do the 49ers move him into the nickel and demote Jimmie Ward? While Ward has been getting dumped on lately, his game against the Arizona Cardinals was not nearly as bad as some would think. The 49ers will face a decision at some point, but for now, they've got a solid cornerback in the lineup.