I had this happen to Fred McGriff between the 2002 & 2003 seasons. In 2002, his numbers were a little worse than his 2001 numbers, but still solid. .292/.367/.450, with 25 homers and 121 RBI's. His numbers took a sharp downturn, and he's now just an expensive pine rider. In 77 appearances, he's hit .229/.302/.310, with 6 dingers and 29 ribbies.
Thankfully, Hee Seop Choi has lessened the blow since he was promoted from AAA. He's hitting .317 with 7 homers(the first two came in his first game) in the 31 games he's been in so far. I think he has the job until further notice.