Sandoval isn't a great third baseman, but he should be solid, and he certainly should have been an upgrade over what we had last year. Finding someone to play LF wouldn't be that difficult. Finding another RH power bat to replace Hanley would be.
As far as the competence of Cherington goes, I don't think he's as incompetent as people are making him out to be. When the team stinks as badly as it does, of course the GM is going to shoulder the blame, whether deserved or not.