Sewald had his contract selected from Triple-A Las Vegas on Saturday, Antony DiComo of MLB.com reports.
With the Mets' bullpen getting used heavily through the first week of the season, Sewald was immediately thrust into action Saturday night and gave up two runs on three hits over 0.2 innings in his big-league debut. The 26-year-old's stay in the majors could be a brief one.