Nava could be an ideal 4th OF. Switch hitter, can probably play LF or RF, good stick, minimal ego, high OBP. Check, check, check, check, check.
That said, I could see Nava's upside approaching that of Werth and Crawford without anywhere close to the cost and it is the exact type of Theo-Sox-Nation-Anxiety-Provoking move that would lead into next year with questions like "Is the Red Sox offense going to be enough with Daniel Nava as the starting OF? I don't think so."