That per the Hardball Times:
Not everyone understands DER, but if you simply subtract DER from one, you'll get a number very similar to Batting Average on Balls in Play. So Tampa Bay's opponents are batting .346 on balls in play (one minus .654); the major league offensive BABIP leaders are Detroit and Atlanta at .320. That's a .26 point difference.
DER isn't a perfect fielding metric. Obviously, pitchers play a role in it too, by giving up more or fewer line drives or infield pops. But, as noted on THT's team page (did I mention it's updated daily?), Tampa Bay's pitchers are actually allowing more catchable balls than the average staff. That means that the fault for Tampa's awful DER lies entirely, and then some, with the team's fielders.
In fact, if Tampa Bay finished the season with their current DER, it would be the worst performance in major league history. By my count, the lowest DER of all time belongs to the 1930 Phillies (.658). And the Phillies were an outlier. The next-lowest DER is about twenty points higher (the 1997 Athletics, at .676).
Admittedly, these historic comparisons aren't completely fair. My historic DER formulas are a little different than the current one because of data issues. If I adjust Tampa Bay's DER accordingly, they do finish higher than the 1930 Phillies. And DER has certainly risen and fallen over the years, as documented in this article.
But given that the Devil Rays' fielders are solely responsible for their low team DER, I'm going to say it anyway: They're the worst fielding team in baseball history.
You know who would help fix that? John McDonald (or Omar Vizquel.)