Given Darvish's performance history, I would not consider it "improbable" that he will yet be effective, even highly effective, this season. Seems to me it could easily go either way.
In any event if the other starters pitch like they have recently (granted, that is a very large "if") I am not convinced a successfull season is necessarily dependent on Darvish getting back on track. Obviously success would be way more likely with Darvish pitching anywhere near his potential.