Last night, Dellin Betances became the new Yankee record holder for most strikeouts in a single season by a relief pitcher with 132. He passed Mo to do it after entering the night tied with him at 130, and considering that Betances needed 87.2 innings to set the new record compared to the 107.2 Mo needed in 1996, I think it's safe to say that it's quite the accomplishment.
With the Yankee record now in his possession and 11 games remaining in the season, the focus shifts to where Betances will end up on MLB's all-time single-season reliever strikeout list. The 132 currently puts him tied for 15th with Duane Ward of the '91 Blue Jays, and looking ahead he's got a pretty good shot to crack the top 10.
The all-time record is held by Dick Radatz, who struck out 181 batters in 157.0 relief innings pitched for the 1964 Red Sox. That''s obviously unattainable, but the rest of the list not so much. The current 10th place holder is Carlos Marmol, whose 138 strikeouts in 2010 made him the most recent entry into the all-time top 10. The only other member of that group from the 2000s is Brad Lidge, who had 157 K for the Astros in 2004 (42.6% K rate).
Betances has made 67 appearances in the Yankees' 151 games played this season. At that rate, he should make another 4 appearances over the final 11 to give himself 71 for the year. With an average of about 1.31 IP per appearance, he'll be good for about 5 more innings this year, and at about 1.5 K/IP, he should be good for another 7+ strikeouts. That would put him at at least 139 for the year, good enough to bypass Marmol (and 2003 Eric Gagne with 137) for entry into the top 10. If Joe decides to use him a little more (I can't see why he would) or Betances strikes out the side a few times, he could inch up into the 7-9 range at 141-144 total strikeouts.
That would be a hell of an accomplishment for a guy who was converted from a failed starter to a full-time reliever last year and wasn't even guaranteed to get a 25-man roster spot when spring camp opened this year. What Betances has already done this season is incredible and historic. What he could end up doing would be even better and will be something fun to pay attention to over these final 11 games.