Well you're guaranteed to have Ross, Crawford, and Ellsbury. Personally, I say keep Nava and Sweeney, but that really limits you when you face LHP.
Lillibridge isn't exactly stellar vs LHP either, but his versatility will probably keep him over one of Sweeney or Nava.
Sweeney may be the kind of a piece that the Sox use as a complimentary piece in a deal for a SP, though.