Lamar Johnson can retire now as Mets hitting coach: The team is undefeated in his tenure. But that is more a function of the Mets bullpen than any nuggets Johnson might have imparted on the lineup in his debut Tuesday. Vic Black and Jenrry Mejia bailed out a stagnant lineup by combining for 3¹/₃ scoreless innings in the Mets’ 4-2 victory over the Pirates at Citi Field. Johnson, who replaced fired Dave Hudgens as hitting coach, was careful before the game to downplay any effect he might have on a Mets lineup that hasn’t scored more than five runs in a nine-inning game at Citi Field since April 5. “These guys here, they are professional hitters — they have been playing this game a long time so they know their swing,” Johnson said. “I’m just here as another set of eyes to help them with things that they can’t see.” The Mets (23-28) left 10 runners on base and were 3-for-16 (.188) with runners in scoring position . With the Mets bullpen thin, Mejia worked two scoreless innings for his fourth save. Mejia’s big pitch came in the eighth, when he got pinch-hitter Pedro Alvarez to hit into an inning-ending double play with the tying runs on base. “Now I’m ready for any situation that comes up,” said Mejia, who last week pitched back-to-back games for the first time since moving from the rotation to bullpen. “I don’t worry about it.” There was a heartbeat from the Mets in the sixth, when Juan Lagares stroked an RBI single to give the Mets a 3-2 lead and Daniel Murphy’s ensuing run-scoring double against Jeanmar Gomez added insurance.