The move had been reported prior to the game, with Walker being removed from the starting lineup shortly before first pitch, but couldn’t be announced until both teams had concluded their games.
Alderson said the player coming back probably wouldn’t be identified for some time. He declined to reveal the parameters of the selection.
“It came together over a period of days,” said Alderson.
Walker was owed about $4.7 million for the remainder of the season.
“Money figures into every trade discussion,” said Alderson. “Whatever the market dictates, you have to react.”
The 31-year-old second baseman is a free agent at the end of the season, Walker is hitting .264 with 10 home runs and 36 RBIs in his second season in New York. He hit .282 with 23 home runs and 55 RBIs last year.
Walker had been rumored to be headed to the crosstown Yankees before medical issues reportedly halted the deal, according to Newsday.
Walker spent the first seven years of his career with the Pittsburgh Pirates before being traded to the Mets in December 2015 for pitcher Jon Neise. He is a .272 career hitter with 126 home runs.
“He was an excellent player,” said Alderson. “He had some injury issues last year and this year as well, but he’s a first-rate guy, very important in the clubhouse. I’m glad we had him.”
The Mets will make a roster move prior to Sunday’s 1:35 finale in Philadelphia. Alderson said it’s possible the player could be a pitcher.
The Mets have vacillated between carrying four bench players and five recently. Jose Reyes – who replaced Walker in Saturday’s lineup – and Wilmer Flores will likely play second base with Walker gone and Alderson said Triple-A infielders Matt Reynolds and Gavin Cecchini could be options down the road.
The Mets, who have made five trades since July 27, might not be done.
“I still have inventory,” said Alderson.