UPDATE 10:22: The Pirates confirm that they have acquired Stewart for a player to be named, who I assume won't be anyone significant. To make room on the 40-man, they've designated McKenry for assignment.
UPDATE 10:07: Ken Rosenthal confirms that the Pirates will deal for Stewart.
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The Pirates are close to acquiring Chris Stewart from the Yankees, ESPN's Buster Olney reports.
The Yankees were very likely to non-tender Stewart today, given that they have Brian McCann and have said they'll tender Francisco Cervelli, and they have a couple other cheap backup options besides. That means Stewart won't cost the Pirates much of anything.
What's less clear is what the Pirates will do with him. As far as I know, he's out of options, and he only hit .211/.293/.272 last year. He's a good defensive player (although he struggled with wild pitches and passed balls last year), and the Pirates may figure there's enough there between his defense and his pitch-framing to justify keeping him as their backup over Tony Sanchez, who has options remaining. (Stewart's pitch-framing is highly-regarded, and pitch-framing is one of the reasons the Pirates acquired Russell Martin last year.) MLBTR projects Stewart will only get a million dollars in arbitration, too, so trading for him now doesn't preclude the Pirates from cutting him later.
In any case, if the Pirates trade for Stewart, they will almost certainly non-tender Michael McKenry.