Brian Cashman is on record saying that he doesn't see a six man rotation in the Yankees' future. Given the current state of the team, it's hard not to blame him. It's not 2003 anymore, and the Yankees don't have the sort of rotation where four or five starters are a threat to throw 200+ innings and squeeze out a young fireballer.
If you've got three or four great starters, it makes sense to maximize their starts, if they can handle the strain. The move from a five man rotation to a six man rotation shouldn't be understated; it means five or so fewer starts for your ace, and means giving a combined 27 starts to the sixth best guy on the team. When that means taking five starts away from a Masahiro Tanaka, and giving them to a Chris Capuano or Adam Warren, you can see how big that gap is. I remain convinced that five extra Tanaka starts in 2014 would have put the Yankees in the postseason, so I won't argue too much with anyone who wants to argue the merits of the five man rotation. But we've already talked about what a relentless spring the Yankees face: no off-days in four of the first eight weeks; including a stretch of 30 games in 31 days in April and May. From everything we've heard out of camp, we should expect to see spot starters in that stretch to give the Yankees' main five an extra day off, to try to help keep them fresh for the long season to come.
A hundred years of baseball tradition suggests that pitchers these days aren't tough enough, don't throw enough, and just need to man up and eat their Wheaties. In Living on the Black, John Feinstein wrote about Tom Glavine's minor league encounter with "arm soreness," cured by Johnny Sain's prescription of more pitching. But Glavine was still just 18 years old. Tanaka (and other Japanese imports) has been pitching on the schedule he's grown accustomed to for nearly a decade. If you believe that you play the way you practice, then it's hard to ignore that Tanaka has spent far longer practicing to pitch once a week than every fifth day.
And the Yankees have the opposite problem from those 2003 Yankees and their deck of aces; with four fragile starters coming off major injuries in Tanaka, CC Sabathia, Ivan Nova and Michael Pineda, with Nathan Eovaldi making the transition to the AL East, and with Warren trying to make the transition to the rotation after a successful season in the pen, the gap between their fifth and sixth starters isn't all that large. Last year, only Eovaldi came close to the 200 innings mark, and even he didn't quite make it (199.2 IP, and if he gives up hits in the AL like he did in the NL, there's no way Joe Girardi lets him get that many innings in).
After that first run, the Yankees have stretches of at least 14 games in 14 days in June, August, and to close the season in September/October. We haven't heard the end of the six man rotation, and we probably won't hear the end of the suggestion unless Tanaka hits the DL.