Walker was removed from Tuesday's start in Atlanta due to back spasms, Anthony DiComo of MLB.com reports.
Walker allowed a hit and two walks and didn't record a strikeout over two scoreless innings before being lifted from Tuesday's contest. It's the second consecutive unplanned bullpen game for the Mets, as Monday's starter, Carlos Carrasco (oblique), also made an early exit due to an injury. The severity of the back issue remains unclear, leaving Walker's availability for his next turn through the rotation up in the air.