DeMeco Ryans did a phenomenal job in his first season as a defensive coordinator, considering the 49ers defense gave up the third fewest yards in the NFL last season.
But it wasn't the defense Ryans envisioned before the start of the season.
When Ryans took over from Robert Saleh, who is now the head coach of the New York Jets, Ryans wanted to make the 49ers defense much more aggressive than it had been under Saleh. Saleh is more conservative by nature and tends to prefer zone coverage, which is designed to take away big plays. Unfortunately, zone coverage tends to give up lots of little completions.
Coming into last season, Ryan wanted to change things up and call bump-and-run man-to-man coverage, which makes sense, because the 49ers' ferocious pass rush forces opposing offenses to throw quick passes. Zone coverage concedes quick passes, while bump-and-run man-to-man coverage takes them away and forces the quarterback to hold the ball longer while receivers work to get open downfield.
At least in theory.
Ryans quickly learned last season that his cornerbacks weren't good enough to play press man coverage. When they did, they often committed penalties or gave up long completions. So roughly halfway through the season, Ryans made the mature decision and told his cornerbacks to line up nine yards off the line of scrimmage and keep everything in front of them. This is why the 49ers defense got burned so frequently on third down -- the corners were more concerned about giving up touchdowns than first downs.
Now, Ryans has Charvarius Ward, a premium cornerback, plus Jason Verrett is returning from a torn ACL. It's likely Ryans will go back to the aggressive man-to-man coverage he wanted to call last season. And if the 49ers cornerbacks don't commit penalty after penalty, the third-down defense should improve tremendously.