From Will Carroll:

It's not looking good for Rich Harden. The oft-injured pitcher is reportedly headed for Tommy John surgery, ending his season and damaging the hopes of the Athletics, though there is no official confirmation. The team is celebrating its sweep of the Yankees, but behind the scenes, the team is trying to plan out how it will deal with the loss of their co-ace. Harden left just four innings into his return from the DL, injuring his elbow in a return from what is now being reported as an avulsion of his oblique rather than the previously indicated lower back injury. (I wonder what the A's put on the Standard Form when Harden was placed on the DL.) I'm not ready to compare Harden to someone like Steve Karsay or even Mark Prior, but it's interesting to note the rapid increase in pitching injuries for the A's over the past two seasons. No matter what the prognosis on Harden, it won't make the A's any more (or less) likely to deal Barry Zito.

If Harden is going to have T-J surgery, that is a pretty huge blow for the A's, both for 2006 and 2007 (since he's not likely to be back until the second half of 2007, if even then).

The A's, of course, have a lot of pitching depth, so it isn't as if they'll be looking at digging up Scott Erickson or Pedro Astacio or Andy Benes to try to fill Harden's spot in the rotation, but Harden is one of the best pitchers in the league, when healthy.  There's going to be a big dropoff from him to whomever takes his place.