Sometimes, the role a player is thrust into isn't the best fit.
Where Buchholz is concerned, first we should define what a "starting pitcher" is. Since there are five pitchers in the starting rotation, what you want out of a starter can vary. If a pitcher is in the bottom end of the rotation, you expect them to log 170-190 innings pitched and keep the team in the game for 65-75% of their starts. But if the starter is a top of the rotation ace, then you must depend on them to pitch over 200 innings and keep the team in over 90% of the games they start.
Given these definitions, we can reasonably claim that a start should be reliable enough to pitch at least 170 innings a season. Clay Buchholz has had five seasons to accomplish this most modest of goals for a starting pitch, yet he's only pitched over 170 innings in a season twice. This season is Buchholz's sixth chance to accomplish this goal, but I'm doubtful he will do it.
Observing this objectively, without bias, one can only surmise that Buchholz has not been a dependable starter for the Red Sox.
We could ask "Why?" and partake in endless amounts of speculation regarding Buchholz's failure to be a normal starting pitcher, nevermind the staff ace that we all know he has the talent to be, but I don't see a point to undertaking that exercise. We have to focus on what is and is not, and Buchholz is not a reliable starter.
I'm not sure what role Buchholz should have for the Red Sox, but it must be one where he's not forced to try and pitch 170-200+ innings. Should he serve as a longman/spot starter out of the bullpen? Possibly, since that's a common position for failed starting positions. Personally, I'd love it if the Red Sox experimented with making him a closer; especially since this organization has had so much trouble finding a reliable closer after letting Papelbon walk.
Some off your will scream at this suggestion, pointing to the fact that Buchholz doesn't have a loss this season and his ERA is under 2.00. The potential for greatness is there -- and I agree. We've all seen it this season, just like we all saw it in 2007. But Buchholz is still on the disabled list; a spot he occupied in 2012, 2011, and 2010. Fact is Buchholz has not been a successful starter at the Major League level. The Sox have a few pitching prospects who will be ready for MLB level soon, so the time for the Sox to define what Buchholz's role is -- and whether he'll ever become a dependable starter -- is now.