There are a few players the Yankees could pursue to fill their need for a starting pitcher, and three pitchers that, by contrast, I would suggest they not go after. Let's start by separating those two groups from each other.
Pitchers the Yankees should leave alone:
1. Cole Hamels
2. David Price
3. Cliff Lee
Pitchers the Yankees should go after:
1. Bartolo Colon
2. James Shields
3. Ian Kennedy
Okay, so what's wrong with the first group? Cole Hamels is having a good season, but he's owed waaaayyyyyy too much money. His contract calls for $22.5 million per year from 2015 through 2018, along with two different options for 2019.
The Yankees can pay someone else that much in the off-season without having to give up prospects.
Price? He's young, he's not owed a ton of money, and he has a good strikeout-to-walk ratio ... but he is probably going to cost a lot in prospects. In a New York Times article dated July 1, 2014, Jorge Arangure Jr. wrote of the Yankees trying to get Price, "they might not have the high-caliber prospects who would make a deal possible."
And Lee's coming off an injury, which should be a red flag to anyone.
So why might Colon, Kennedy, and Shields be good fits for the Yankees?
None of them would likely be very expensive, which is a plus.
Each would also provide a statistical upgrade to the rotation. Here are the fielding independent pitching (FIP) and strikeout-to-walk ratios (K-BB) of Colon, Kennedy, and Shields compared to David Phelps, Shane Green and Chase Whitley. They're written like slash lines for batters, like this: Player last name: FIP/K-BB ratio.
As you can see, the three players I said the Yankees should target are ahead in the majority of these stats. That's why I think New York should go after them. They're all doing well and could help the team.
And should things go poorly, none of the three players are either owed all that much money or have a lengthy contract, so it wouldn't be a disaster. But let's hope that if the Yanks do trade for any of these players, that things do go well and they end up helping the team.