Since Carlos Beltran hit the trade market, Mets general manager Sandy Alderson has been waiting for someone to offer him a top prospect in return. Alderson’s waiting game paid off Wednesday afternoon, when the Mets agreed to a trade with the Giants for pitching prospect Zack Wheeler.

It looked like the Mets wouldn’t be able to get a high-level prospect in exchange for Beltran because of a lack of leverage. Not only is Beltran set to be a free agent at the end of the year, but he’s a Scott Boras client (meaning he will almost certainly test the free-agent market before agreeing to a contract), had a full no-trade clause, and whatever team acquired him wouldn’t be able to offer him arbitration, so a team wouldn’t get anything for him if he left after the season.

But Wheeler is exactly the type of high-level prospect the Mets were looking for in exchange for Beltran. The sixth overall pick in the 2009 draft, the Georgia native is a hard-throwing right-hander, with a fastball that ranges from 92-97 mph. After entering the season ranked as the No. 55 prospect in baseball, according to Baseball America, the 21-year-old shot up to 35th in BA’s midseason rankings that came out earlier this month.

Wheeler is 7-5 with a 3.99 ERA in 16 starts for the Giants’ High-A affiliate, striking out 98 and walking 47 in 88 innings. There are concerns going forward about his control, but Wheeler’s strikeout rate shows he has the power stuff, along with the ability to miss bats, needed to be a true top of the rotation starter. He is similar to the Mets’ top prospect, Matt Harvey, who ranked 30th overall on BA’s midseason rankings.

Given how the deck was stacked against the Mets when they began shopping Beltran around the league, Alderson couldn’t have hoped for much more than Wheeler. Now Harvey and Wheeler give the Mets have a pair of prospects who have the potential to pitch at the top of their rotation for years, and help push them back to the top of the National League East.

tbontemps@nypost.com