I think Beltre may be OK too, but if Lowell *is* still here, and he's as healthy as all that, I'm going to want Beltre to prove effective before he's named the presumptive starter. If for no other reason than to establish Lowell's trade value.
These player controversies have a way of working themselves out, and we have room for all of Lowell, Beltre, Youkilis and Ortiz on the 25-man roster right now. Since there isn't a ton to choose from between Lowell and Beltre when they're both healthy and it's mostly just a question of different skillsets (Beltre's got better range, power and athleticism, Lowell's got better positioning, hits for better average, and has some of the softest hands of any 3B in history) I think that it's very likely that the two will split 3B fairly evenly at first, until the play of the two men on the field establishes what needs to happen next.