Yankees starters have had the following ERA by month:
April: 3.41
May: 3.94
June: 4.49
July: 3.27
You gloss over those stats and think they're A-OK, but here's the problem:
Yankee rotation, peripherals by month:
April: 1.20 WHIP, 7.0 K/9, 2.02 K/BB
May: 1.29 WHIP, 7.1 K/9, 2.51 K/BB
June: 1.29 WHIP, 8.0 K/9, 2.39 K/BB
July: 1.31 WHIP, 6.5 K/9, 2.21 K/BB
Not coincidentially, while the ERA for July is low, that has a lot to do with the strength of the schedule, because the peripherals were below the benchmark set by the starters for the rest of the year, and it coincides with Hughes' recent ineffectivenes, and Pettite's injuries.
The fact of the matter is, that Javy Vasquez, Andy Pettite (due to the recent injury), Hughes and Burnett, are currently all Wild-Cards, and the Yankees needed a starter to cement their "Big three" for the playoffs, because the assumption that one (or even two) of the above mentioned starters were bound to get their act together. The fact is, the rotation has struggled, no use denying it, and as Y228 pointed out, they weren't going to dangle their top prospect for a rental, get turned down, then go after another starter, if they didn't think adding said starter was necessary.