He'll bounce back soon hopefully....I can't imagine the rush of emotions he must have had hitting his 500th on Father's Day with his dad in attendance. Like you said, it's just a "hangover" so to speak. He'll end up with a .270-.280 avg. His days of hitting .300 are definitely over.
My team BA has gone from .283 to .277 over the past 4 days...and in large part due to Griffey and Burrell.