You often hear it said, as Favre said to Jennings, "80% of you is better than 100% of anybody else."
From a physical standpoint, that's not true. It certainly wasn't true of Polamalu in the Super Bowl, and it appears to be untrue of Matthews in the last 2 games, a guy who looks to be bothered by something.
In the NFL, the difference in physical attributes between an impact player and an ordinary one are quite small. For instance we talk about 2/10 of a second over 40 yards as a wide gulf, when in fact it's 2 blinks of the eye. If a dinged up player loses those two blinks, he's just not the same player. Nelson, for instance, is a 1 blink, maybe 1.5 shy of where he was in his long speed, where the jets come on at 15 - 20 yards. When it comes to quick twitch blinks at the OLB position, the difference between impact and ordinary is even more subtle.
But here's the thing. There are important intangibles of having a dinged impact player who is not himself physically out on the field. Assignment reliability, leadership, chemistry, setting an example. Maybe he can't go every series, but you want him out there for however much he can stand.