I know what you're thinking, "but Troy you told us Jacoby Ellsbury was a horrible center fielder!" I've spent a lot of bandwidth giving my opinion on Ellsbury, but I think I'm going to take one more swing at it (much like Ellsbury would do instead of taking a pitch).
Ellsbury is not a horrible baseball player as evidenced by his 5.4 WAR totaled in 2008 and 2009 combined, which is above average, but not outstanding. Awhile ago I suggested Ellsbury as the option to replace Jason Bay in left field and move Josh Reddick to center. This has picked up steam recently and Michael Silverman wrote about it in the Boston Herald this past week.
The second hold-up is positional adjustment. This is something we haven't talked about much yet, but it is part of the WAR calculation. Obviously each position has a different group of peers and you must be compared to them. We'll cover this more later, but the central part here is center field gets a +2.5 adjustment for 162 games and left field gets a -7.5 adjustment. Ellsbury will lose a full 10 runs or 1 WAR just by moving to left field.
Next up is his defense in left field. He has a UZR/150 of 21.8 in left field, but based on only 80 defensive games. There is a lot of noise there and what he would actually do is a big question. If he did total a 20.0 UZR in left field he would be a 4 WAR player in left. If he continued the poor fielding of 2009 and totalled a UZR lower than -10 he would be a significant liability. Suddenly you have poor defense, but without the huge LF bat to make up for it.
Lastly, I am starting to open myself to the possibility that Ellsbury's 2009 defense was in part due to Bay being so poor. If he had to shade to left to help out he would possibly be allowing more hits to fall toward right. While this may just be a way to deflect his 2009 UZR/150 in center, we should consider his -16.5 UZR might have been a bit extreme. We'd see if Ellsbury UZR would average out only if the Sox added someone like Matt Holliday in left who is at least average with his glove.
My ultimate goal with Ellsbury would be a positive UZR in 2010, but even a 0.0 would be a gain. I would also like to see him bat near the bottom of the order where his speed can cause more havoc, especially as long as his OBP is below .360. Two early projections have him performing very differently. ZiPs has him posting a .344 OBP in 2010 and Bill James gives him a .360. The .360 is still not elite, but could qualify him higher in the order, and is an encouraging sign of growth.
If this all were to happen he gives you a ~3 WAR player in center worthy of $13 million dollars. If he ended up in left field and had average defense there he drops to near 2 WAR. That is what he was worth this year meaning the center fielder would need to be a 3.6 WAR to make up for the loss of Bay.
I really can't see any transactional combination that would make this move beneficial to the Red Sox. There is no free agent center fielders worth noting this offseason leaving this whole thing as just a highly unlikely long shot.
I don't trust UZR in Fenway, which has so many tricky angles, not to mention the green monster. These make players look a lot worse than they probablty are in reality.