A six-man rotation is, in most cases, inefficient. The quality of starting pitchers tends to diminish the further down the depth charts you go -- Boston could barely scrape together fourth- and fifth-starters in 2011, for instance -- so giving starts to a sixth man that take away innings from more talented pitchers is often a dead-end plan.
The Red Sox find themselves in a specific situation that could merit a temporary switch to that setup, though. Alex Speier brought up this point on Sunday, stating that the Red Sox have 20 games in 20 days in May, starting with their series against the Orioles this Friday, and ending with those same O's down in Baltimore on May 23.
The schedule looks something like this, assuming the rotation remains in place the way it's currently set-up:
That's four starts a piece for each Red Sox starter, in a 20-day stretch with no days off giving any of the pitchers extra days of rest, or allowing Bard or Doubront to be skipped in favor of moving Jon Lester up on the schedule, as they did with the April 22 rain out against the Yankees.
This is where Aaron Cook comes in. On Tuesday, Cook's first opt-out in his contract becomes active, and he can leave the Red Sox if he's not on the major-league roster. The thing is, the Red Sox can't just shove him in a relief role in order to make sure his opt-out won't be used, for a few reasons. First, Boston and Cook negotiated a deal in which Cook would be given a chance to get healthy and start. If the situation never arose, he would be free to pursue that same gig elsewhere. The Red Sox have the ability to just bring Cook up and use him as they wish, but doing so would not reflect well on them, something that agents would remember in future negotiations. It might not sound like a significant thing, but next time they need an Aaron Cook-type contract, problems could arise if there are other suitors.
Second, it's not 100 percent clear what chance Cook's shoulder has of staying healthy in a relief role. Pitching coach Bob McClure says that it might be possible, but that's not much of a medical vote of confidence.
Felix Doubront and Daniel Bard have pitched far too well to be removed from the rotation just to avoid losing Cook. In fact, if it came down to straight-up losing Cook in favor of keeping the two young hurlers in the rotation for the year, that would be tolerable. It wouldn't be the most optimal use of a pitcher who induces grounders and has a history of getting outs when he's in one piece, though, and figuring out what does fit that description is the team's job for the rest of Monday.
If Bard, Doubront, and the rest of the rotation can't have natural days off in a 20-game stretch, Cook could be used to create them. A six-man rotation in this 20-day stretch, starting with Lester on day one once more:
Rather than four each, it's just Lester and Beckett getting four starts (and with a few extra days of rest thrown in along the way). Bard and Doubront are both reduced to three starts in this stretch, and the Red Sox get a look at Cook against major-league hitters, in order to see if keeping him around past his June 1 opt-out is something that makes sense for the club. If it turns out that there is no permanent home in the rotation for Cook, and a bullpen role won't work, the Red Sox wouldn't have to wait much longer than that to be able to trade him, should Cook be amicable to that arrangement. (With Cook's permission, he could be dealt earlier than mid-June, when players signed this past off-season are first allowed to be traded.)
There's no need to go to a six-man rotation all season long. They have 16 games in 16 days to close out June and begin July, 17 to finish July and get them through half of August, and 16 again between late August and early September, but in between, it's business as usual on the schedule. Daisuke Matsuzaka will be back from his rehabilitation assignment at the next no-rest junction, meaning Cook is useful, but optional, assuming health in the rotation. The trade deadline and Roy Oswalt are still out there, too, should Dice-K's stint as cavalry be more Custer than Roosevelt. Regardless of who it is in the role, the Red Sox could use those points to give Bard and Doubront more rest than they might otherwise get, without interfering with the rotation as much as a six-man unit normally would.
The need to keep Cook from opting out is lessened once Dice-K returns, even with these similar situations in the future, but in the now, when Dice-K remains in rehab and Cook's opt-out looms, switching to a six-man rotation makes sense for a Red Sox team with young pitchers whose arms they need to protect. This is a team both transitioning to 2013 while simultaneously competing, and bringing Cook up not to replace, but to supplement the current starters, is a move that allows them to continue walking that line between the future and today with their rotation.