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 veteran Mets’ second baseman, whose medical reports came under scrutiny in the aftermath of the spat between the bickering rivals. “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 general manager Sandy Alderson on Thursday cited a deal that fell apart minutes with the Yankees just before the trade deadline.

Though Alderson did not mention the name of the player involved, sources revealed later that Walker had been part of a deal, which fizzled because of the Yankees’ concerns about his medical report.

Such public revelations are especially damaging for the 31-year-old Walker, 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.