There are two great players who have been on my mind as I've watched Jorge Posada struggle through this season. Both are outfielders, one was a switch hitter and the other was a Hall of Famer.
Bernie Williams was one of the best CF in the game from '98-'02, when he averaged 24 HR and 105 RBI each season to compliment his.326/.411/.538 line.
When a 2003 knee injury robbed his power, his already creaky shoulders couldn't compensate. The previous five years, he'd managed a wRC+ in the 140s (well above average), and for the final four years of his career he was just barely average (100). That's a bigger than 25% decline in production due to injury and age.
From 1963-1967, Roberto Clemente put up an impressive .332/.376/.502 line. From 1969 until his death in 1972, he hit .339/.387/.521. An impressive decade, except for 1968 when he hit .291/.355/.482. If there was ever a season and a reason to say "bad luck" it's Clemente in '68. At 33, his skills weren't really in decline yet, though I'm sure that was the fear in Pittsburgh. And yet, in one year (admittedly, the original "Year of the Pitcher"), his offensive output dropped about 10%.
I think of these two examples because I think Jorge Posada is suffering from both problems at once. In the last 5 years, Posada has separated himself from the pack less than Bernie Williams did, posting a 128 OPS+ in that span and only a 118 mark the last 3 seasons. In that context, the fall to a 70 mark isn't as far- roughly 40%.
Posada insists he isn't injured, but years behind the plate obviously haven't been kind to him (and he's not on the same diet plan as Pudge Rodriguez). 23 hits after 42 games won't cut it. It'll be interesting to see just how much time the old man has left.