Wouldn't it make sense to pursue FA SP's? You only have to pay a lot of money for them, as opposed to giving up prospects AND THEN pay a lot of money for them.
Sox have like 10 outfielders on their roster right now, they have to move some of them. Now, if you are talking trading for Hamels or signing Lester, signing Lester is a no brainer since the money you would pay each is pretty close. I think if they they can get a solid young arm for Cespedes they have to do it though. Their current roster configuration makes it all but certain they are making trades before opening day.
I can't see them getting a solid young arm for Cespedes, as he is a free agent after this year. I can see them getting Latos who also is a free agent.
Moving Cespedes frees up $ allowing them to spend it somewhere else.