Everything I read also says mid- to late-April. According to Will Carroll at BP, the surgery was minor, and the recovery is straightforward. The only concern, apparently, is "how much arm strength and conditioning he'll lose during the recovery period," especially considering the early season timing of the layoff.
Carroll says that, given Lowry's history of "control issues," expect a little extra time before he gets back to his regular form. As I understand it, that's not really a consequence of the surgery, but of missing training days.
I do not expect big things from Lowry, but he is probably worth keeping until his return. With any surgery to alleviate repetitive stress injuries, I think there is always a chance he comes back much better than before.
Fight racism in our schools. American Indians are people, not mascots.