Adrian Peterson has a torn meniscus, which is expected to keep him out until at least December. Given Peterson’s age — he’s 31, with his half-birthday coming yesterday — it’s reasonable to start thinking about whether the end is near for Peterson.

In his last 8 games (excluding playoffs), he rushed 150 times for 529 yards (3.52) with 5 TDs, one fumble, and 26 first downs. Prior to this stretch, Peterson had averaged 159 carries, 786 yards (4.95), 6.5 TDs, 2.24 fumbles, and 37 first downs. Given his performance in the one playoff game during that stretch (23 for 45 with one fumble), including that would only make the numbers look worse.

I came up with a relatively simple adjusted rushing yards metric to measure running back performance (note that in the formula below, rushing TDs are actually worth 20 yards because all rushing TDs are also rushing first downs):

(rushing yards + rushing first downs * 9 + rushing TDs * 11 – fumbles *30) minus (rush attempts * 5)

This is basically a mix of rush efficiency and rush quantity: we take rushing yards, add some other information, and then provide a penalty for each attempt used. The graph below shows Peterson’s game-by-game results for his career, along with, in black, a trailing-8-game average:

Peterson led the NFL in rushing last year, but he did not finish the season strong and played poorly in the playoffs. Then, his first two games this year had been disappointing, too. Here’s a look at his last 18 regular season games — i.e., all of his games since the start of the 2015 season:

Player performance is volatile, as you can see in Peterson’s first chart. So it’s always tough to know whether an older player in a slump is declining due to age or just experiencing random variance (to say nothing of a player’s statistical decline due to a weaker supporting cast or a tougher slate of opponents). Still, Peterson has finished with a sub-2.00 yards per carry average just 8 times in his 127-game career, and three of those games were his last three games (including playoffs). Had Peterson not been injured, we would have been hearing questions about his productivity soon, and as early as this week. And while losing Peterson is a big blow on paper, it’s far from clear whether the Minnesota offense will actually be worse without the version of Peterson that was likely to show up over the rest of the season, anyway.