A golden opportunity in the name of Starlin Castro has sprung seemingly out of nowhere, and the Mets need to do everything they can to take advantage of it. If that means trading Zack Wheeler and perhaps even another pitcher to get the best-hitting shortstop this side of Troy Tulowitzki, so be it. On Saturday, however, there was no indication the Mets were moving quickly to investigate what it would take to get the 24-year-old Castro, after the Cubs’ trade of pitchers Jeff Samardzija and Jason Hammel for prospect Addison Russell on Friday created a shortstop overload. One Mets source said the chance of making such a trade was more likely in the offseason, and the consensus among baseball people was that Cubs president Theo Epstein would prefer to wait until then to sort out his trio of shortstops, which now includes top prospects Russell and Javier Baez as well as Castro. But why wait? Neither team is going anywhere this season, and the Mets, in particular, desperately need to show their fans they are moving in the right direction, adding offense — and payroll — that could help them contend as early as next season. For the Cubs, meanwhile, it makes sense to trade Castro for the pitching they’ll need to complement the impressive array of position-player prospects that Epstein has accumulated during his rebuilding job. And the Mets are “a perfect match,” as one member of the organization put it on Saturday. They’ve been biding their time, waiting for a chance to trade some of their young pitching for a difference-making hitter, and at a time in baseball when there are few, if any, available, the Mets need to seize the moment. Will GM Sandy Alderson do it? “Knowing Sandy, I think he’ll try,” said a baseball person who has worked for Alderson in the past. “I’ve seen him get aggressive, more aggressive than you might think, when he sees a chance to get something he wants.” Alderson himself offered no hint of interest at Citi Field on Saturday. Commenting on Castro would be grounds for tampering, anyway, but when asked about the value of having impact offense at shortstop, he smiled and said the Mets are very excited about Matt Reynolds.