One of the biggest question marks surrounding the NFL's push for an expanded season of 18 games involves injuries. NFL players already suffer far too many injuries, especially gruesome ligament tears, bone breakages, and serious concussions. Adding two more games would seemingly exacerbate this problem.
However, there have been few real studies of injuries that give us any idea of what to expect out of two more games. Plus, just looking at all injuries isn't very helpful. Every player gets nicked up, and sometimes misses a game or two here or there during the season (think Donovan McNabb circa September 2009). Those aren't the injuries that people are worried about with two more games on the schedule.
The real question is season-ending, potentially career-ending, injuries. How much more serious risk would the NFL be subjecting these players to?
To begin to answer this question I went through the NFL's transaction records for last season and isolated when players were placed on injured reserve (IR), signaling that their injuries were too severe to return to the field for the rest of the year. Separated by week of the season, here's what we get:
Since you're talking about small sample sizes and basically (bad) luck, the numbers are going to fluctuate from week to week. But overall there's a huge upswing of players getting placed on injured reserve as the season goes on. The first five weeks of the season, no more than 10 players in a week were shelved for the season. But once you get to the eighth week, you never have less than 10, and sometimes more than 20 big injuries.
The data's certainly not perfect. Sometimes a player can be injured well before he's placed on the injured reserve list, since it takes time to evaluate and come to the conclusion that the player can't help the rest of the season. Perhaps a bigger problem though is that as the season goes on, lesser injuries can cause a player to go on IR. In other words, once the last few weeks of the season come around, a player who's likely to miss a month may not be worth keeping on the active roster.
Especially for that last reason, this projection of about 55 more season-ending injuries in an 18-game season seems overly pessimistic. So what if we went to a model that tried to eliminate those issues?
For this second graph I did two important things. First, I took three-week averages of IR transactions rather than individual weeks. Short of compiling multiple season's worth of data, this is the best method we have to mitigate the volatility of these injuries. Second, I stopped looking at injuries after 10 weeks. That's somewhat arbitrary, but it should prevent us from counting minor injuries that put players on injured reserve with just a few weeks left. Any player who's likely to miss 7 or more games is probably going to be put on IR permanently, no matter when the injury occurs.
Even accounting for these two factors, trips to the IR still rise over the course of the season (though not as rapidly as before). If serious injuries increase at this admittedly crudely-calculated rate, you're looking at approximately 40 more "season-ending" type injuries — more than one per team — in an expanded regular season.
Whether that's more than you're willing to accept for two more weeks of NFL football is up to you.