J.D. Martinez is expected to make his long-awaited New York Mets debut on Friday when the team opens a seven-game homestand against the St. Louis Cardinals.Signed to a one-year, reported $12 million contract on March 23, Martinez wasn’t in spring training with the Mets. He needed time to prepare for the season, then dealt with a back issue.In a pair of games at Triple-A Syracuse, Martinez was 3-for-8 with three runs driven in.With the Mets, Martinez is expected to bat behind Pete Alonso at designated hitter.Manager Carlos Mendoza said earlier this week that barring a setback, Martinez would be in the lineup Friday. He will not be the everyday DH from the start.”I’ll have a conversation with him, but out of the gate, I’m not gonna run this guy and DH him every day,” Mendoza said. “He’s gonna need some days, you know. I think it’s one of those where, like I said, I’ll sit down with him and put together a game plan here because it’s important. He’s too valuable for this team and we gotta take care of him.”