The Red Sox have added another arm to the mix. This one a former Yankee with a career 3.21 ERA in 126 major league innings, most of which cam in a relief role.

Up until this offseason, 28-year-old Alfredo Aceves had spent his entire professional career with the New York Yankees franchise until signing a major league contract worth $650K with the Red Sox. The Sox will have Aceves under arbitration control for the next three seasons.

So what did the Sox get exactly?

As mostly a reliever, Aceves found success in the big leagues back in 2008 with 30 innings pitched and in 2009 when he posted 1.3 WAR in 84 innings. However, the Red Sox plan to convert Aceves to a starter, which means his already below average strikeout rate (major league career 6.6 K/9 and minor league career 6.5 K/9) would probably regress or at least not break the 7 K/9 mark. While the strikeout potential is quite low if Aceves becomes a starter, his control and command will likely play the biggest role in his future success or failure. In 640 minor league innings, Aceves posted a 2.5 BB/9. In his last healthy season–we can throw out his 2010 numbers due to a back injury–Aceves posted an excellent 1.7 BB/9 with the Yankees.

If Aceves can keep his walks down and get hitters to chase his “pitcher’s pitches”–he produced a 30 percent chase rate in 2009, which was about five percent better than the league average–there is at least some chance he could turn into a league average starting pitcher. Even if that doesn’t work out, he can always be converted back into a reliever to try and add even more depth to the pen.

Now, let’s not get too carried away here. While there are certainly some optimism that Aceves will be a valuable big league arm once again, we must also consider the lack of upside in his game.

As stated above, Aceves isn’t going to be a big strikeout threat, so he’ll rely heavily on the results of his balls in play. In that 2009 season, Aceves’s success was likely due at least in some part to a particularly low .249 BABIP. He is also more of a fly ball pitcher than ground ball pitcher, which could lead to a spike in home runs as a starter. In 2008 and 2009, he allowed over one home run per nine innings both at triple-A and the majors.

All-in-all this is a no-lose move for the Sox. They only dish out $650K for 2010 and gain a pitcher that has at least some sort of upside to help the big league club.

It will be interesting to see how Aceves transitions to his new role for his former team’s arch nemesis.