Flexen was optioned to Triple-A Syracuse on Tuesday, Anthony DiComo of MLB.com reports.
Flexen joined the big club in mid-June and posted a 3.68 ERA with nine punchouts over 7.1 innings before being demoted. Jeurys Familia (shoulder) and Luis Avilan (elbow) were activated from the injured list, while Luis Guillorme was recalled from Triple-A in the Mets' flurry of roster moves.