Jeff Nelson criticized the Mariners' front office last week after the team failed to make a deal at the trade deadline. The 36-year-old right-hander pitched for the Yankees from 1996-2000.
The Yankees acquired Benitez in a trade with the New York Mets on July 16.
The deal comes less than a week after the trade deadline, meaning both players had to clear waivers before being dealt.
Nelson was 3-2 with a 3.35 ERA and seven saves in 46 games for the Mariners.
After saving 21 games for the Mets this season, Benitez was 1-1 with a 1.93 ERA for the Yankees.