Walker threw two perfect innings Monday night at Philadelphia in his season debut, then was pulled after 30 pitches. He was replaced by left-hander David Peterson, who is likely to fill in for Walker in the rotation.
Walker was 7-11 with a 4.47 ERA last season, his first with the Mets, and was picked for the NL All-Star team. A strong first half was followed by a terrible second half, and he had a clean-up procedure on his knee in the offseason. The knee gave him a little trouble late in spring training, but he was able to take his turn the first time through the rotation.
The Mets already were missing two-time NL Cy Young Award winner Jacob deGrom. He was sidelined before Opening Day because of inflammation around his shoulder blade, a problem that could keep him from pitching in the majors until June.
In other roster moves, the Mets activated closer Edwin Díaz from the bereavement list and sent right-hander Yennsy Díaz outright to Triple-A Syracuse.