Patrick Willis missed over two full games with a groin injury.
The success of the San Francisco 49ers defense against the run has declined significantly, from 2012 to 2013.
In 2012, the 49ers ranked fourth in the league, allowing only 94.2 yards per game on the ground. The defense also ranked third in yards per carry, allowing an average of only 3.7 yards. Opponents rushed for seven touchdowns all season.
The 49ers defense against the run has weakened, at least statistically. In 2013, they rank 18th, allowing 106.7 yards per game. They are currently 17th in the league, allowing a 4.0 yards-per-carry average. Through seven weeks, the 49ers have already allowed seven touchdowns on the ground, equaling what they allowed all of last year.
So, what is the problem? Has the 49ers defense declined that much?
The answer is that there really is no problem. On the contrary, the 49ers defense has been playing very well in their past four games, all wins.
The defense did play poorly against the run in their two losses. Seattle's Marshawn Lynch rushed for 98 yards as the Seahawks amassed a total of 172 yards on the ground. The Indianapolis Colts fared even better, rushing for 184 yards, led by Ahmad Bradshaw's 95 yards.
The issue for the 49ers has not been one of scheme or strength. The main factor is that injuries to the 49ers' front seven have hit the defense pretty hard.
The 49ers lost starting nose tackle Ian Williams in the second game of the season. His replacement Glenn Dorsey has been battling a hamstring problem. Injuries to defensive tackles Justin Smith and Ray McDonald have limited their practice time and, in the case of McDonald, has noticeably hindered his play.
All-Pro Linebacker Patrick Willis has missed time due to injury and starting outside linebacker Aldon Smith has entered a rehab facility to deal with a substance-abuse problem. Smith has missed the last four games.
Only starters NaVorro Bowman and Ahmad Brooks have stayed healthy. Whenever a defense is playing with subs or with players who are playing hurt, it's bound to have a negative result in performance.
That is exactly what has happened to the 49ers.
Looking forward, the key for the 49ers defense is to stay healthy. In addition, those players who are out or nursing nagging injuries must come back strong. Only Williams and defensive back Chris Culliver are out for the season.
The 49ers depth on defense has enabled them to overcome injuries, continue to play well and win. If they can stay healthy, the 49ers still have one of the best defenses in the NFL.
All team defense stats courtesy of NFL.com.