Now, the idea of going back and tossing out all of the crappy Dustin Moseley/Javier Vazquez, etc. starts and inserting what Haren did with the Angels, while tempting, has a spectacular amount of flaws. There are so many presuppositions that need to be made, and obviously the rotation would have continued to evolve and adapt based on how well/poorly certain guys were pitching, so it's impossible to know exactly whose numbers he'd be replacing. There's also obviously no way of knowing whether Haren would've pitched exactly the same way he pitched for the Yankees in his Angels' outings, and also whether the Yankee offense would've performed the same way it did in the Moseley/Vazquez/Nova starts.
Another significant problem with re-imagining the second half of the season with Haren in pinstripes is that we don't know what the Yankees would have ended up having to yield to the D-Backs to complete the trade. All I remember hearing was that the deal was rumored to be centered around Joba Chamberlain. According to this July 24 Jon Heyman article, the Yankees actually balked when Arizona asked the team to include Joba, which seems questionable to me. If Arizona was indeed interested in a package headlined by Joba and a couple of lower-level minor leaguers, and Cash said no, then that could go down as one of Cashman's worst non-trades, especially given that Arizona settled for Joe Saunders and three
In any event, this being the offseason, where hypothetical scenarios and wild speculation rule the day, I thought it'd be fun to at least try and get a general idea of how a Haren acquisition might have affected the team down the stretch, and even into the postseason. Before we even look at the numbers, I think it's pretty safe to say that the Yankees probably would've been a better team in the second half and also held onto the AL East crown if they had had Dan Haren toeing the rubber every fifth day. The team could've replaced Joba's bullpen innings with someone from the minor leagues -- perhaps Ivan Nova, who might've fared even better had he been pressed into relief duty instead of starting -- and even though Joba's peripherals ended up being quite good on the season (2.98 FIP, 13th-best among AL relievers), I don't think too many fans would've missed him given his penchant for surrendering timely hits to the opposition in 2010.
In any event, the best we can do is take a look at the performances of the guys that basically ended up taking over for Andy Pettitte from the time a Haren trade would've been consummated (let's assume July 25 would've also been the day the Yankees acquired him), and compare them with what Haren actually did, as Haren would've presumably inherited Andy's rotation slot:
Conveniently enough, Moseley and Nova ended up combining for almost the exact same number of innings that Haren gave Los Angeles -- though it took them two more starts to do so -- which helps make this comparison slightly more apt.
As you can see, those are some mighty impressive numbers from Mr. Haren, and as Phil noted in his comment, Haren also pitched to a 1.64 ERA against the Rangers over three starts while holding them to a .190 BA.
Now, would Haren have pitched to a 2.87 ERA in a Yankee uniform? Probably not, but I think it's safe to say that had the Yankees been able to swap out Moseley/Nova's .788 OPS against and 4.98 ERA with Haren's .648 OPS against and an ERA around two runs lower, the team almost certainly would've fared much better in the second half of the season.
In addition to improving the Yankees' lackluster finish to the regular season, adding Dan Haren might have had even more significant repercussions on the postseason. Though we couldn't have asked for a better result in the ALDS against the Twins, Haren would have presumably taken the hill in Game 2, pushing Pettitte to Game 3 in the first round, and perhaps bumping Hughes all the way to the next round, assuming the Yankees still swept.
An ALCS rotation of CC Sabathia-Dan Haren-Phil Hughes-Andy Pettitte looks a touch more formidable than Sabathia-Hughes-Pettitte-A.J. Burnett, wouldn't you say? Haren almost certainly would've pitched better than Hughes did in Game 2 (although then again, so could your grandmother), but we don't even know if they would've played the Rangers in the LCS, because they might have won the division with Haren. And again, this is all with the benefit of extraordinary hindsight, as it also assumes that Pettitte would've still been able to stay in the rotation and pitch as effectively as he did through the injuries we didn't find out about until after the Yankees had been bounced from the postseason.
Of course, given what we know now about Sabathia himself pitching with a minor injury, this postulating is probably all moot anyway -- even if they made it to the World Series, I don't know how far they'd get with Sabathia battling a torn meniscus.
Anyway, while I think Phil has a pretty good point in that the Yankees missing out on Haren might have changed the course of the season, it's ultimately impossible to pass judgment on this non-deal as we don't know what the parameters of a potential trade were. If the transaction was indeed for Joba and assorted minor league friends not named Montero, then Cashman probably will end up regretting passing on a 30-year-old starter with 31.1 career fWAR for a slightly-better-than-he-appears-to-be reliever with mostly untapped starting potential and who looks like he's going to be permanently relegated to that great discard heap in the Bronx.