While there is obviously no clear-cut answer, the Yankees' best option actually is currently working out of their bullpen but, as previously stated, is not Phil Hughes. New York acquired Alfredo Aceves as a free agent in the offseason before the 2008 campaign. A starter who spent three fairly successful seasons in the Mexican League, Aceves got his first chance to start for the Yankees in 2008 as a replacement for Darrell Rasner. He was very good in '08, starting four games for the Yanks and appearing in 30 innings in total. During his short stint up with the club, Aceves was highly effective, posting a 2.40 ERA with a 1.167 WHIP. He allowed only 8 runs on 25 hits and finished the season with a 1-0 record in six appearances.
Not all was right with Aceves though, as his peripheral stats show. His 10 walks and 16 strikeouts led to a sub-par 1.60 K/BB ratio and a 4.8 K/9 mark, both indicators that the ERA and WHIP might possibly have been deceiving, especially considering his BABIP was only .233. But in 2009 Aceves has elevated his game, taking strong strides in improving those numbers. In about the same sample size — 36 innings leading into Sunday's win — Aceves had struck out 29 batters (7.3 K/9) and walked only seven (4.14 K/BB). He had also shown solid improvements in ERA (2.25), WHIP (1.00) and his record (5-1).
While some of the same draw backs that apply to Hughes also apply to Aceves (i.e. not being stretched out, eliminating a valuable bullpen piece) with Alfredo neither of these factors apply to the degree that they do with Hughes. While it is true that Aceves hasn't started much this season (aside from four starts at AAA) he did finish last season as an effective major league starter, was a high volume innings guy for three seasons in the Mexican League, and has had 12 appearances already this season in which he has pitched two or more innings in a game, including four when he worked three or more innings.
And while it would undoubtedly hurt to lose a quality arm like Aceves' out of the 'pen, he — unlike Hughes — is not being groomed as the Yankees' setup man. The loss of a middle relief pitcher, regardless of how good he is, can be more easily weathered than the loss of a successful setup man. Aceves is arguably one of the best middle relief pitchers in baseball this season but he also has the stuff to be one of the better fifth starters, which is a far greater need for the Yankees at this point in their season. And if the Yanks are that high on Sergio Mitre, or still believe in Kei Igawa enough not to cut him outright, it would more make sense to bring one of them in as a middle relief replacement, although it seems the newly recalled Jonathan Albaladejo will fill that role for now.