At a staff meeting a couple of days ago, Sandy Alderson told Mets executives and baseball personnel that he strongly believes that the ballclub can and maybe even should win 90 games this season. Yes, 90. And, according to people who were in the room, after Alderson’s decree prompted discussion about how to best maximize the Mets’ assets, owner Fred Wilpon chimed in at one point with his own pointed assessment. “We better win 90,” he said, according to a source. What, Matt Harvey’s elbow surgery was just a cruel practical joke of some kind, and he’s secretly preparing for Opening Day? Otherwise the idea sounds preposterous, right? A Las Vegas sports book, Bovada, on Wednesday released its over/under proposition bets, and it put the number for the Mets at 73½. And that was before the ballclub announced that Jon Niese was sent back to New York for an MRI after experiencing what Terry Collins described as a “dead arm.” Both Niese and the Mets said they didn’t think it was anything serious, but you know how that goes with pitching injuries — not to mention the Mets. In other words, expect the worst.