Ask a Sox fan "What's the biggest problem for the 2009 Red Sox?" and you'll get a variety of answers: Ortiz's swing, Lester and Beckett's struggles, anything that Julio Lugo does. But arguably the worst performance has been teamwide: the Sox, with a few exceptions, are playing atrocious defense.
How bad is the D? One quick and dirty measure of team defense is "Defensive Efficiency," which you can find on Baseball Prospectus. Defensive Efficiency tracks the rate at which balls put into play are turned into outs; a good defense will record a higher number. By this measure, the Red Sox have a .676 rating, 'good' for 11th in the league and 24th in all of baseball. [Aside: the league leader in Def Eff is the Detroit Tigers, at .706, followed by the Blue Jays at .705.] This is quite a downgrade from last season, when the Sox finished 4th in the league in Def Eff.
Another metric which you can use to evaluate defense is UZR, which is now available free on Fangraphs. Fielders and teams can have either positive or negative UZRs. Unfortunately, by this measure, the Sox again rate as one of the worst teams in the game: -11 runs for the season thus far, which makes them the third worst AL team in UZR. Meanwhile, the Rays are leading the game, at 22.9, followed in the AL by the Rangers (16.8) and Tigers (14.4).
Looking at the UZR numbers for individual Red Sox players, a distressing picture emerges. Of the starters, only Pedroia (2.4), Youk (1.6) and Drew (1.9) have decent ratings. Ellsbury, for all his amazing catches, is surprisingly mediocre (-0.2). Lowell is a huge disappointment at third base (-2.4). Lugo's defense has been abyssmal (-4.2), and Green hasn't been much better (-1.8). But the real butcher has been Jason Bay, who is -6.4 in left field, even as he is saving the team with his bat.
This sieve-like defense is losing games for us both quiety and flagrantly. The latter is exemplified by Nick Green's atrocious two-base error in Seattle (5/17), or Lugo failing to execute a double play in Dice-K's game against Santana and the Mets. But games are lost quietly too, in the many times when a ball gets by Lowell, or Bay's tentative play allows a ball to fall for a hit.
I believe poor defensive performances from our fielders is the number #1 problem with this team. Beckett and Lester would have markedly improved numbers if they had had a better defense behind them.
What are the options to address this? At SS, Green could replace Lugo, if you believe the UZR difference is significant enough; also, Lowrie will come back soon, so he might be the ideal solution to fill the yawning abyss at that position. Barring injury, Lowell is the third baseman, so aside from hoping he'll play better, little can be done. Bay really ought to get some days off from fielding, in part because he is so bad, and in part because he deserves some time off (he played 162 games last season, plus the playoffs, and hasn't missed a game all season).
There are undoubtedly some good fielders out there, and Theo should consider a Nomar-like trade if the defensive woes linger. As the season is still young (47 games in), it's possible that the individual and team numbers will improve. The question, as with Ortiz, is how long can we wait?