What's the max that Ellsbury's value could increase, 20% or so? Whatever it is, there remains a less than zero chance that he will get injured and we'll see his trade value crater to zero. Why risk losing it all for a chance at a small increase? The sooner you trade him, the better.
I think you mean "greater than zero," but regardless, note that this statement is just as true of Holland as Ellsbury.
Ellsbury's value may be higher if he plays better; but, I think that sentiment is vastly overblown. An extra 80-100 game sample (in a roughly 600 game career thus far) will change a projection, but not as dramatically as this implies.
I think this evaluation fails to look at the story arc. Ellsbury starts with a red-hot September at 23, then has a so-so age 24 year followed by a promising, but not overwhelming age 25 year. A freak injury shelves him for most of his age 26 year. Then at age 27 he explodes with a superstar-level performance that confirms all the promise of that first cup of coffee, and then some.
So far, the trend is clearly upward--interrupted, but upward. A year ago, I think the consensus was that while we had probably just seen his best year, he had established a new normal, and that normal was an above-average-to-excellent two-way player at a skill position.
Then he gets hurt again--and unlike the first time, he comes back in time to play for a while. Badly. It seems likely that the poor offensive performance in the last third of 2012 was due to aftereffects of the injury; but even assuming this is true, we don't know for certain that those aftereffects are temporary, or how long they might last.
So the first two-thirds of 2013 is no ordinary 80-100 game stretch for Ellsbury. It's a chance for him to either re-establish his status as one of the best CFs in the league, or brand himself as damaged goods. The potential swing in value is huge. Cashing him in this winter may be the safe bet, but if you're bullish on his recovery, it's selling low.