We now know who that player is: Single-A pitcher Eric Hanhold.
RHP Eric Hanhold has been sent to the Mets as the player to be named in the August 12 trade for INF Neil Walker and cash.— Milwaukee Brewers (@Brewers) September 12, 2017
Hanhold was a 6th-round pick by the Brewers out of the University of Florida in 2015. He's struggled since then, putting up a 5.01 ERA in 201.1 innings across three levels. He did fare better this season after working mostly in the bullpen, though, putting up a 3.94 ERA as a 23-year-old in Single-A, striking out 60 in 64 innings. He still allowed 71 hits, but some of that might have been the result of some bad defensive luck. He did put up a GB/FB ratio of 1.9.
Considering how the Mets' trade of Walker was always going to be more of a salary dump than anything, this type of return makes sense. The Mets get an okay bullpen prospect who might be getting a little old for his level in the minors and got out of paying Walker for the rest of the year. The Brewers got a solidifying bat in a lineup that needed a hitter of his type.
Walker has hit .268/.388/.465 in 23 games with the Brewers, including 3 home runs and 5 doubles, mostly splitting his time between second base and first base.
Statistics courtesy of Baseball-Reference