They are both assumptions and both can be true, but players frequently have off years and adjustment years, while guys permanently losing all ability to play at age 31 is not that common.
As far an Nixon, most of his injuries have come recently, playing no more than 134 games since 2003. Drew on the other hand has not had that many injuries as of late (one trip on the DL since 2003, due to getting drilled in the wrist). Drew's extensive injury history all happened before 2003, while most of Nixon's happened since 2003. If Drew were perfectly healthy in his days in St Louis, but injured all the time in Atlanta and LA than I'd be worried that injuries had caught up with him.