1. That Alonso will hit better than Heisey, Gomes, Nix, Frazier, Sappelt and anybody you might have reasonably acquired (plus, possibly, Dorn and Francisco).
2. That Alonso will hit well enough to hold down first base in a league where his counterparts will include Pujols, Howard, Adrian Gonzalez and Fielder, for starters.
3. That neither Alonso at 1B nor Votto in LF will represent a significant drop in defense.
I'm not sure I've seen enough to warrant taking on all those uncertainties.