Newest Marlin Javier Vazquez won't be remembered as the worst Yankees pitcher all-time, but he comes pretty close to the top (or bottom) of the list when you look over the variety of nuggets we found on Baseball-Reference.com, Fangraphs.com, and from Stats LLC.
Vazquez finishes his Yankees career with a 5.09 ERA, fifth-worst in franchise history among those who made at least 50 career starts with the team. The all-time worst is the immortal Andy Hawkins, 5.21.
Vazquez and Kenny Rogers are the only two Yankees pitchers to make at least 50 starts, have a record over .500, and an ERA of 5.00 or worse.
The problem with Vazquez was that he got worse as the season went along. He had a 6.92 ERA after the 2004 All-Star Break and a 6.64 ERA after the same point in 2010. Dating back to 1952 (as far back as these statistical resources can go), his 6.80 ERA post-break ERA is a full point worse than any other Yankee. He even made Hideki Irabu look good.
Vazquez didn't do any damage to his postseason ledger this time around. He currently holds the Yankees postseason record for worst postseason ERA, with all the harm coming in 2004. His 9.53 tops the next man on the list (Don Gullett, 7.98) by a considerable margin.
The advanced metrics don't help him much. Vazquez posted a 5.56 FIP (basically an ERA-equivalent stat), meaning his combo of strikeouts, walks, and home runs allowed rated third-worst among anyone who threw at least 100 big league innings this season. And he and Scott Kazmir were the only two pitchers in the majors with at least 150 innings pitched and a negative Wins Above Replacement (meaning the Yankees would have been better off starting a "quadruple-A" level" minor leaguer)
But on a positive note, the guy somehow managed a winning record. Vazquez, despite his struggles, continued a streak of winning 10+ games that now stretches through 11 straight seasons, the longest current streak for any pitcher in the majors. Some numbers can be a little deceiving.