Walker (side) was placed on the 10-day injured list Wednesday, Tim Britton of The Athletic reports.
Walker exited Monday's start against Atlanta due to left side tightness, and he's reportedly felt good recently. However, the team will elect to place him on the injured list to avoid a long-term injury. It's not yet clear how much time Walker will miss, but the Mets could utilize a four-man rotation in the immediate future since they have a day off Thursday.