Well, that's not really the reason. In a perfect world, Francona would love for him to start every fifth day for the next four weeks. However, it is clear that Boston has their priorities straight in terms of protecting their best prospect's right arm (from FanHouse)"
"It's been proven with young pitchers if you're increasing their (yearly) increments too much, you're putting them unnecessarily at risk," [Terry] Francona said. "We don't want to do that."
Last year, at the age of 21, Buchholz threw 119 innings over 24 starts in A ball. Here's his line so far this year.
So that's 141 innings so far this year, 22 more than '06. Now another Francona quote from this Yahoo article:
The Red Sox have to do what they're doing right now. You can't be putting more innings on this kid's arm with a six game lead and 99.988% chance of making the playoffs. That wouldn't make any sense.
But October is a whole different animal. The difference between advancing and getting eliminated is so tiny, teams can't afford to hold anything back. And if they don't have Buchholz in their rotation, the Red Sox would be doing exactly that. From Nate Silver's fascinating Unfiltered post on BP:
Silver goes on to explain just how significant a 4% jump is- it's the equivalent of adding a five win player for two season, which would probably cost a team about $25MM.
Looking at it in this light, it seems like it would be insane for the Red Sox to not start Buchholz in October just to save a few innings from his arm. It seems like a risk they have to take. And maybe they will. Regardless of what their plans are for October, this is what they would have done. Either way, it's not a good risk to have Buchholz starting games in September for a team with a comfortable lead. But taking advantage of an asset like Buchholz in the playoffs is a necessity. Hopefully, Theo Epstein has been reading BP (entirely possible), and we'll get to see Buchholz's encore about four weeks from now.
Stats taken from Baseball-Reference (now with minor league stats!). Pictures taken from here and here.