If there was any uncertainty about the final spot in the rotation, Daisuke Matsuzaka seemed to clear it up Monday. A day after Jenrry Mejia had a strong five-inning showing, the apparent front-runner Matsuzaka blanked the St. Louis Cardinals into the seventh inning. That seemingly locked in Matsuzaka as a member of the Mets’ rotation. Mets officials huddled after Monday’s 5-3 win to cement their plans. An official announcement should come no later than Tuesday. Because of Matsuzaka’s considerable big-league service time, and with him being on a minor-league contract, the Mets must inform Matsuzaka by noon Tuesday whether he has made the Opening Day roster. Otherwise, they must give him a $100,000 retention bonus to send him to the minors, or cut him loose. “I think I’ve done pretty much everything I can and shown what I can do out there,” Matsuzaka said through interpreter Jeff Cutler. “The decision is up to them now and we’ll see what happens.”