Yeah 2003 was bad, but that was 4 years ago and he didn't have near as good of a lineup around him. He's had bad months littering his career and the difference between him hitting .313 last year and .283 the year before was how many of those he has.
If you expected and counted on .310 + when you drafted him, then you should worry. Those of you that drafted him for a .280 - .290 BA and 35-40 HR shouldn't worry much. I'd just bench him now if you have another alternative. If Sheffield serves as your backup 1B, welcome to the crub