Walker had been scratched from the lineup 10 minutes before the start of the Mets' 3-1 loss to the Phillies game at Citizens Bank Park on Saturday.
Milwaukee's second-base contingent, which consisted primarily of Jonathan Villar and Eric Sogard, ranks last in the league with a .672 OPS going into Saturday's game.
Walker, 31, hit .264 with 10 home runs and 36 RBIs in 73 games with New York this season. He has a career .272/.339/.437 slash line over nine seasons with the Pirates and Mets. New York acquired Walker from Pittsburgh in a trade for pitcher Jon Niese in December 2015.
Walker was taking batting practice with his New York teammates two hours before Saturday's game when Mets general manager Sandy Alderson came out and conferred with manager Terry Collins behind the batting cage.
A few minutes later, Walker and Alderson left the field for the clubhouse.
The Brewers beat the Reds, 6-5 to remain in third place in the NL Central division, two games behind Cincinnati and co-leader Chicago.
The Mets are a disappointing 52-62 and third in the NL East, and they've recently begun selling off veteran pieces. They traded closer Addison Reed to the Boston Red Sox and sent first baseman Lucas Duda to the Tampa Bay Rays before the July 31 non-waiver trade deadline, and they sent outfielder Jay Bruce to the Cleveland Indians for minor league pitcher Ryder Ryan on Wednesday.
The New York Yankees were reportedly close to acquiring Walker in a trade before the deadline, only to back out because of concerns over Walker's medical reports. Walker underwent season-ending back surgery last September and missed 36 games this year with a partially torn hamstring, but he told reporters this week that his health is not a concern.