He takes the place of the drek that took over for Monty. It is quintessential to win the division, so they don't have to play Russian roulette with the wildcard playoff game. Every trip around the rotation with and automatic loss set the Yankees behind the Sox. Now, they get a veteran to keep them in the game. So when the Yankees play the remaining 10 games with the Sox, they don't have to win 9 of 10 to just get back into the race.
A fifth (or fourth) starter is not the Yankees’ top priority here. They need bodies in the lineup who can give them some timely hitting. They’ve been held to 2 or fewer runs 11 times since June 1st ... and most of those games were against non-descript starters, not Cy Young Award contenders.