PHILADELPHIA — Neil Walker isn’t happy about becoming collateral damage in the latest round of mudslinging between the Mets and Yankees, a development that could prove costly during his upcoming free agency.

“It’s disappointing,” said the second baseman, whose medical reports came under scrutiny after the spat between the clubs. “More than anything, it’s disappointing.”

Jay Bruce’s trade to the Indians instead of the Yankees — who believed they made a competitive offer — raised questions about whether the Mets have a prohibition about doing business with their crosstown rivals. In response, Mets GM Sandy Alderson cited a deal with the Yankees that fell apart just before the trade deadline.

Though Alderson did not mention the name of the player involved, sources later revealed that Walker had been part of a deal that fizzled because of the Yankees’ concerns about his medical report. Such public revelations are especially damaging for Walker, 31, who is approaching free agency.

“That’s the piece of it that’s not really fair to the player,” he said.

Last season, Walker underwent back surgery. On June 15, he went on the DL with a hamstring injury that sidelined him until three days before the deadline. Walker nevertheless drew last-minute interest.

“Whether information was leaked or whatever the case may be, I don’t know where things went wrong,” Walker said. “I’m here, I’m happy to be a Met right now. But to hear that I’m not healthy enough to be traded but I’m healthy enough to be in the lineup every day is a bit confusing. I don’t know where things got lost.”

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