The point of interest for A's fans is the defensive falloff from Bobby Crosby to Marco Scutaro when Crosby was injured. Crosby's numbers for 2005 were among the top 10 defensive shortstops in the majors, while Scutaro's were the second-worst, ahead of only Derek Jeter.
Now keeping in mind that we have to take these defensive numbers with a healthy grain of salt, as Dave Cameron so eloquently explains at USS Mariner, it's a fun exercise to try to roughly quantify how much Crosby's injuries impacted the A's last year.
Scutaro made 229 outs at shortstop. If you replace his opportunities with Crosby's rate numbers, you'd get 257.4 outs. So replacing Crosby with Scutaro cost the A's about 28.4 outs on defense.
How many runs is that? Using Chris Dial's run value per play at shortstop of .753, that means those extra outs costs the A's about 21.4 runs.
Using the rule of thumb that 10 runs is worth about one win, we can then estimate that Crosby's injuries cost the A's about two games in the standings, just on defense alone.
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On offense, Crosby had 50.8 Runs Created in 84 games (.605 RC/game), while Scutaro had 47.1 Runs Created in 118 games (.399 RC/game). That's a difference of .206 RC/game. If you give Crosby 162 games instead of 84, and subtract out Scutaro, you'd get an extra 16 runs.
Again, going by the 10 runs/win rule of thumb, the A's lost about a game and a half on offense from Crosby's injuries.
So in total, replacing Crosby with Scutaro for half the season cost the A's about three or four games in the standings. That's not enough to make up the seven games they missed the playoffs by, but judging by these (admittedly rough) numbers, it is roughly half the story.