[Scrap Irony]

He doesn't have to be league average and doesn't project to be so.

However, it could certainly happen that he grade out as league average over the course of one season.

1) One season is a small sample in terms of UZR. A couple nice OOZ plays and taking charge between Choo and Ludwick by grabbing all the balls in the gaps that he can get to might help.
2) Familiarity with the position may mean less "vapor locks", as he's played CF pretty much his entire life and knows the position as well as any. No (or very few) errors would boost his UZR as well.
3) Bruce has a great arm. He could very well lead all CFers in assists by a wide margin. (This depends, IMO, on Cespedes' position in Oakland.) That in itself could also boost his defensive WAR by a fairly large margin.

I'm not saying he's a good bet, but it's within the realm of possibility.