Plus, Boston is going nowhere in 2015 regardless of Castillo, Cespedes and any pitcher they add. There are a lot of issues with the Red Sox roster right now..
It starts with the pitching, as none of their young pitchers have been effective in the majors so far, with Workman just getting optioned to AAA after putting up a near 5 ERA (4.21 xFIP, 4.30 SIERA) and some mediocre peripherals. Clay Buchholz is still suffering from Ricky Romero-itis.. and Rubby De La Rosa has been pitching well above his head and is due for regression.. but he has been one of their better pitchers. Allen Webster has long-term bullpen middle reliever written all over him.. his control issues aren't getting any better, and he continues to get lit up.. his SIERA is over 6 and xFIP is an ugly 5.93. And of course, the jury's still out on Anthony Ranaudo, Matt Barnes and Henry Owens, as two of those pitchers have yet to reach the majors, and Ranaudo has put up less-than stellar numbers in the MLB despite the 2-0 record. And Joe Kelly is a backend starter/long reliever at best. Boston has a lot of work to do with that rotation.. one ace starter isn't going to fix this mess.
For 2015, only De La Rosa should be guaranteed a spot, along with Kelly. None of the others, even Buchholz, should be guaranteed a spot on a big-market team like Boston. But I can't see Boston adding more than 1 starter, if any because of the low-balling they did with Jon Lester possibly leaving a sour taste in agent's mouths, and ownership's impetus not to add starters on 5-6 years deals when over 30 due to the risk of the contracts being albatrosses (Sabathia, Verlander, potentially Cain). Oh, and they need to resign Uehara as well.
As for the offense, any improvement must come from David Ortiz and Dustin Pedroia. Especially Pedroia, who needs to show better numbers offensively than what he's had this season(100 points below career norms on OPS.) and is being paid like the .800 OPS hitter he's supposed to be.
They need Bogaerts to develop.. and right now, he should be in AAA right now working on his swing, but is only up because Boston refuses to give up on him due to being a #1 prospect, even though they're better off putting Holt there for the time being. Ditto for Middlebrooks.. he needs to stay healthy and show his power potential. But time is running out for him.. he'll be 27 next season and might be out of options.
And despite what fans think, Stanton is NOT walking through that door.. Miami wants MLB ready young players for Stanton, and will not take AA/AAA prospects that are years away, especially when they're trying to contend.