With the Yankees getting back both David Phelps and Masahiro Tanaka, barring any setbacks anyway, in September does New York have too much pitching to continue to throw out a five man rotation? Would a six man rotation work for the Yankees? In short, yes.
You want to give as much rest to Hiroki Kuroda as you can after two consecutive breakdowns at the end of the season due to fatigue. You want to give as much rest to Masahiro Tanaka as you can so you don't push that elbow and torn ulnar collateral ligament too far too fast. You don't want to toy with David Phelps like the Yankees did with Phil Hughes and Joba Chamberlain and move him from the rotation, to the bullpen and back again. At the same time you don't want to take starts away from your pitchers that are pitching effectively in Shane Greene, Brandon McCarthy, and Michael Pineda. While Pineda might not be hurt by throwing every sixth day instead of fifth with his recent run of injuries you still want that ball in his hand every fifth day.
The part of the team that is helped the most though by this idea is the bullpen. Presumably a pitcher with five days rest instead of four will be able to go longer into games and that pitcher should be fresher, more effective, and have more control and command than usual. More Pineda, Phelps, and Tanaka innings means less innings on Adam Warren and Dellin Betances which can only help.
Let's look at what the Yankees could do in September with the rotation. Just for fun let's assume everyone is back and ready on September 1. I know this isn't realistic but with no definitive dates for a Tanaka or Phelps return at this point it's the best I could do. In parenthesis next to the pitchers name I will put the numbers of days rest he will be pitching on. See below:
9/2: Boston Red Sox vs. Shane Greene (4)
9/3: Boston Red Sox vs. David Phelps (-)
9/4: Boston Red Sox vs. Hiroki Kuroda (5)
9/5: Kansas City Royals vs. Michael Pineda (5)
9/6: Kansas City Royals vs. Brandon McCarthy (5)
9/7: Kansas City Royals vs. Masahiro Tanaka (-)
9/9: Tampa Bay Rays vs. Shane Greene (6)
9/10: Tampa Bay Rays vs. David Phelps (6)
9/11: Tampa Bay Rays vs. Hiroki Kuroda (6)
9/12: Michael Pineda (6) @ Baltimore Orioles Game 1
9/12: Brandon McCarthy (5) @ Baltimore Orioles Game 2
9/13: Masahiro Tanaka (5) @ Baltimore Orioles
9/14: Shane Greene (4) @ Baltimore Orioles
9/15: David Phelps (4) @ Tampa Bay Rays
9/16: Hiroki Kuroda (4) @ Tampa Bay Rays
9/17: Michael Pineda (4) @ Tampa Bay Rays
9/18: Toronto Blue Jays vs. Brandon McCarthy (5)
9/19: Toronto Blue Jays vs. Masahiro Tanaka (5)
9/20: Toronto Blue Jays vs. Shane Greene (5)
9/21: Toronto Blue Jays vs. David Phelps (5)
9/22: Baltimore Orioles vs. Hiroki Kuroda (5)
9/23: Baltimore Orioles vs. Michael Pineda (5)
9/24: Baltimore Orioles vs. Brandon McCarthy (5)
9/25: Baltimore Orioles vs. Masahiro Tanaka (5)
9/26: Shane Greene (5) @ Boston Red Sox
9/27: David Phelps (5) @ Boston Red Sox
9/28: Hiroki Kuroda (5) @ Boston Red Sox
Obviously you can play around a little with that last game of the season and still be in a great position if the Yankees are in the postseason hunt. You could start Michael Pineda on four days rest for the last game of the seasons and still have Brandon McCarthy available in a potential play in game. You would have Masahiro Tanaka either way for a potential Wild Card Game and a well rested Hiroki Kuroda for any potential ALDS. It looks good on paper but you have to get there first.