The Boston Red Sox fell from an estimated 20 Defensive Runs Saved as a team in 2008 to second worst in baseball at -52 Runs Saved last year. This, no doubt, was a key contributing factor for the pitching staff’s increased ERA. Mike Lowell played through a hip injury that turned a previously good defender into an abysmal one, totaling -18 Runs Saved at third. Additionally, young shortstop Jed Lowrie hit the DL early in 2009, forcing Nick Green, Alex Gonzalez, and the defensively-challenged Julio Lugo into the lineup as fill-ins. The team acquired another bat in Victor Martinez, a notoriously bad defensive catcher.

Recognizing their defensive shortcomings, General Manager Theo Epstein has brought in three defensively-minded acquisitions this offseason: Adrian Beltre, Marco Scutaro, and Mike Cameron. Here’s how each performed in 2009 compared to the Red Sox at their respective positions:

Source: Bill James Online

If the new acquisitions manage to repeat their performances from last year, that’s an upgrade of 84 runs on defense alone. Beltre, Scutaro, and Cameron all had defensive seasons consistent with their previous seasons, based on Defensive Runs Saved, so a repeat of 2009 isn’t out of the question.

Additionally, the Red Sox will move centerfielder Jacoby Ellsbury to replace average defender Jason Bay in left field, where Ellsbury’s weak arm can hide and his range will shine. Based on his previous performance at the corner outfield spots, Ellsbury is likely to be a five to ten run defensive improvement over Bay.

Using the rule of thumb that 10 runs equals one win, defense could boost Boston as many as eight or nine wins beyond their 2009 win total. While the Red Sox haven't signed any big name free agent hitters (though they did add pitcher John Lackey), it is our estimation that improved defense alone will easily more than make up for lost offensive production (primarily the loss of Jason Bay).