Breaking down Football Outsiders' 2013 Adjust Games Lost

Injuries are an unfortunate, yet inevitable part of the game. Coaches and players often dismiss them as excuses as every team has to deal with and attempt to overcome them. While there's obviously some truth in that, teams aren't forced to deal with the same degree of injuries on a yearly basis and injuries are often an important piece of the equation when attempting to explain a team's performance.

When attempting to quantify the extent to which teams were affected by injuries, Adjusted Games Lost (AGL) – a metric developed by Football Outsiders – is the most useful measure that I've come across. For those unfamiliar with the metric, there are a couple of important things to note. First, the player's role matters. An injury to a starter obviously hurts a team more than an injury to a bench player receiving 10 snaps per game. And second, players that are playing through injuries – meaning those who are listed on the injury report during the week but still end up playing come game time – are typically playing at less than their true ability level. AGL accounts for both of these factors, producing a number that is far more useful than simply counting the number of games each player missed and adding them together.

Football Outsiders recently published AGL numbers for the 2013 season and the 49ers found themselves in an unfamiliar position on the list. Here's a quick look at San Francisco's AGL numbers since the 2008 season.

After finishing as one of the 10 healthiest teams in football for each of the previous five seasons, the injury bug finally bit the 49ers in 2013. San Francisco's 84.2 AGL were more than they had totaled in the previous three seasons combined. As Scott Kacsmar notes in the FO article, the 68.0 AGL jump from 2012 to 2013 is the third largest increase in their database, which goes back to 2002.

Injuries at the wide receiver position accounted for the largest share of San Francisco's AGL. Only San Diego and Seattle were more injured at the position. Obviously, the loss of Michael Crabtree for the first 11 games of the season due to an Achilles injury is the big one here. Mario Manningham and Quinton Patton were also sidelined for significant periods of time.

Despite being forced to trot out Anquan Boldin and a bunch of scrubs at receiver for much of the season, the biggest reason for the massive upswing in AGL last season was a result of a large increase in injuries to the defense. Have a look at the 2012 and 2013 AGL numbers on the defensive side of the ball:

It's well known around these parts that Vic Fangio prefers to forego rotations on the defense, opting to leave his best players on the field for nearly every snap. The first two seasons with Fangio running the show, that strategy mostly worked out in San Francisco's favor. As you can see in the table above, the 49ers were incredibly healthy in 2012 but were one of the most injured defenses in football in 2013. We saw Fangio use more of a rotation along the defensive line last season, but perhaps that was more due to necessity than a change in preference.

Ian Williams, who suffered a broken ankle in Week 2 and missed the remainder of the season, was the most significant injury up front. While no one else along the defensive line – outside of rookies Tank Carradine and Quinton Dial – was out for an extended stretch of time, both Justin Smith and Ray McDonald spent large portions of the season playing despite being listed as questionable. It's worth noting that while the questionable tag is supposed to mean the player has a 50 percent chance of playing, as long as we're at the mercy of the team being responsible for reporting injuries that's probably never going to be quite accurate. Regardless, with the number of times Smith and McDonald were listed as questionable, it's fair to say that they were probably "working through something."

In the secondary, the ACL injury suffered by Chris Culliver prior to the start of the season accounts for much of the 22.8 AGL contributed by that unit. Tarell Brown missed three games with a rib injury while Carlos Rogers and Eric Reid each took the field for three games in which they received a questionable or doubtful listing.

So what does this mean for the 49ers' prospects of good health in 2014? Well, despite a higher correlation in recent years, the year-to-year AGL correlation still isn't very high. This means that 2013's poor AGL isn't very predictive and is unlikely to carry over to next season. After all of the negative regression talk that surrounded San Francisco going into the 2012 season, I'm sure that it's a word that leaves a bad taste in the mouth of some fans, but this is a situation in which the 49ers are likely to see some positive regression. We can never truly predict injuries, but considering San Francisco's strong organizational focus on preventing injuries, along with the aforementioned massive jump in injuries last season, it seems fair to expect the 49ers to return to being a healthier than average team in 2014. Given how tight the race for the NFC West crown figures to be, a little better luck on the injury front may be the slight edge that pushes the 49ers towards recapturing the division title.