Walker (shoulder) threw a bullpen session Sunday, Laura Albanese of Newsday reports.
Although Walker was able to throw a bullpen after playing catch Thursday and Friday, manager Buck Showalter said afterward that the right-hander will likely require more than one rehab start prior to rejoining the major-league club. The Mets haven't yet revealed when his rehab assignment will begin, but it's possible that he'll be able to return to the majors sometime in early May.