Unless one of the Red Sox pitching prospects happens to evolve in an unexpected way, there is no future Jon Lester on the farm. The Red Sox do have pitching prospects, however, enough that they can round out a big-league rotation with some of them in 2015, and trade quantity for quality with another team looking to unload their expensive (or about to be expensive) high-end arm. There are options, if nothing else.
The Red Sox do not have a power bat in the system, so it always made sense to attempt to turn Jon Lester into one once it became clear that Boston wasn't going to re-sign him, at least not in-season.* Cespedes might not be an on-base machine, but he has tremendous power and has spent his career to this point in a pitcher's park that shares a division with two more extreme, pitcher-friendly environments. Now, Cespedes comes to the AL East and Fenway Park, both of which will treat him well: you shouldn't throw his previous line away, but given some time, you'll likely forget it existed.
*The chances the Red Sox re-sign Lester are almost nonexistent, but they are also non-zero, so the slimmest possible caveats abound for the moment.
The reason the Red Sox have held on to Will Middlebrooks through his struggles -- besides his youth and paycheck -- is his 34 homers in 190 career games in spite of said struggles. The Sox do not have a right-handed option with Middlebrooks' power that they control beyond 2015 elsewhere, at least not until Xander Bogaerts proves that he's actually going to be a 25-30 homer player. Their minor-league leader in homers is Triple-A first baseman Travis Shaw, who just hit his 20th of the season and is even less of a sure thing than Middlebrooks. Rafael Devers, who could be a force in a lineup someday, is 17 years old and in short-season ball. Mookie Betts should be a productive bat, but he's not going to be a power hitter: like many Red Sox prospects, walks and doubles will be his game.
Red Sox trade Lester to A's for Cespedes
The Red Sox have finally dealt their ace Jon Lester, signaling the official push for 2015 and beyond.
That's not a criticism of those prospects: Betts, Garin Cecchini, and Blake Swihart could all be major pieces of future Red Sox lineups, maybe as soon as in 2015. Having hitters who can drive all those guys on base in is a significant help, though, especially when facing pitchers who won't give a team many opportunities to string hits together. That's part of what makes David Ortiz and Mike Napoli so important -- outside of their own ability to draw a walk -- and why, much to the chagrin of many Sox fans, Jonny Gomes kept getting starts in the playoffs last year over Daniel Nava. Without power, men on base get stranded. Without men on base, power is just a bunch of solo homers. Balance is necessary, and the Sox had none.
The Sox actually have an above-average on-base percentage in spite of a below-average batting average, but their power output has been dreadful for a club who plays their home games in a hitter-friendly park: the AL is slugging .395, while the Sox are at just .373: adjust for Fenway, which increases doubles and triples for everyone and homers for right-handers, and that .373 looks even worse. More power is a must, and Cespedes has it.
The Red Sox will need more pitching to compete in 2015, there is no question there. They also needed more hitting, though, and finding that in the system or on the open market was going to be difficult given the dearth of power in both arenas that has led to the Sox holding on so tightly to Middlebrooks to begin with. Bogaerts figuring things out will help, and if Middlebrooks ever settles in at third, there's some more, but a Red Sox lineup without Napoli or Ortiz in it isn't all that far away. Getting Cespedes, who should help now, in the short-term future, and beyond if he's extended, gives the Red Sox something that most of the rest of the league is still looking for, something that they desperately needed.