Dave Cameron over at Fangraphs makes an interesting case for a replacement in left field for Jason Bay. One that would be cheaper, just as valuable, and would not require a long term commitment. His new left fielder for the Red Sox? Mike Cameron.
The key question is how much value to you get out of an excellent center fielder if he plays nearly all his games in left field and one half of those games in Fenway Park. Although I would worry about the Red Sox ability to score runs sporting a league average offensive player in left field, Cameron would certainly help the Red Sox defense, which was a brutal 28th in the majors last year. The Mariner's "3 center fielder outfield" (both in terms of defense and hitting ability) led the AL with a .717 mark, which the Red Sox had a .679. For those unfamiliar with the statistic, defensive efficiency looks at the rate at which balls in play (any non-strikeout, non-walk, non-home run outcome) become outs. It isn't adjusted in any way.
Another thing to keep in mind is that the Red Sox would net two draft picks (1st round and sandwich round) from having another team sign Jason Bay. Cameron, a type B free agent, would not require the team to forfeit any draft picks. Cameron would also give the Red Sox a tremendous amount of roster flexibility in the outfield and wouldn't require them to carry a true backup center fielder.