He most certainly is the starter. The GM said so, the Manager's track record says so and his two year deal says so. If he was merely an "alternative" it wouldn't be a two year deal. There certainly is a level of suck where Taveras can lose the job, but he sucks already and he was brought in to be the starter. He'd have to be Patterson bad to lose the job and given that its a two year deal, probably worse than that in 2009.
There are always alternatves. Laynce Nix and Darnell McDonald are two signed this off-season. Guys are cut loose all the time. Willie Taveras is guaranteed suck.