The hits keep coming for the Mets- and the good kind. Francisco Alvarez left Tuesday’s game against the Tigers with a right knee injury in the bottom of the sixth and will undergo imaging on Wednesday. The catcher was in discomfort following a swing with A.J. Ewing at second base, and after a brief visit from the training staff, Alvarez was removed from the game and replaced by pinch-hitter Luis Torrens, who drew a walk. Alvarez, who’d cooled off this season, drove in a run and doubled earlier in the game against Detroit right-hander Jack Flaherty. The Mets already had plenty of injury concerns before Alvarez went down, as Luis Robert Jr., Jorge Polanco and Francisco Lindor are all regulars who are on the injured list. Robert has been sidelined for two weeks with lumbar spine disc herniation and isn’t close to returning.  President of baseball operations David Stearns said Tuesday that Robert’s back soreness had “not resolved” and the outfielder was seeing additional specialists.…