There is no doubt that Brian Cashman has done a spectacular job of improving the Yankees offense at the trade deadline and giving up very little in the process. His acquisitions of Stephen Drew, Chase Headley and Martin Prado have solidified the bottom of the order. The Yankees now have an actual MLB caliber player at every position. That does not seem like its saying much, but it is the first time in the last two years that's the case.
Cashman did a great job of identifying some buy low candidates. All three players are better players than what they were showing earlier this season and that looks to be the case in a short sample size with the Yankees so far.
Prado is under contract for next season and will be a solid bat in the bottom of the order. Headley and Drew will be considered to be re-signed. The top of the order is strong this year with Brett Gardner having a career year, and Derek Jeter is leading all American League shortstops in on-base percentage (.329). It will be even stronger next year when Gardner and Ellsbury occupy the top two spots in the lineup.
So, with a strong top and bottom of the lineup all the Yankees are missing is one elite slugger. Mark Teixeira has had a nice power season when healthy with 19 home runs and a .217 ISO, but he should not be the player with the most power on your team anymore.
Carlos Beltran has been on fire in the second half hitting .357/.418/.571/.989 with a 175 wRC+. However, he is still 37 years old so you never know when he will start to break down.
Brian McCann is certainly a better hitter than he showed in the first half and is also improving, but he is an above average hitter and not an elite slugger.
Obviously, it is not easy to get a player like that, especially now with teams locking up their young stars early. You pretty much have to develop a player like that at this point to get him in his prime.
The only huge power threat in the 2015 free agent class is Nelson Cruz, who is 2nd in MLB with 29 home runs. However, Cruz is 34 years old and comes with a lot of injury concerns. He has come down to earth in the second half hitting .104/.197/.179/.376 after a first half that was the best of his career.
Believe it or not the Yankees lead the AL in home runs in the second half with 24. They are sixth in wRC+ (101), fifth in wOBA (.323), and fifth in runs (81). So, they have been much improved since the first half, but still not great.
The Yankees have been missing that slugger ever since the decline of Alex Rodriguez. They have yet to replace the production he was giving them in his prime. That player will be very tough to find, but they will not get to the top of MLB until they do.