MLB

Mets had to trade K-Rod away

In his most important player transaction so far as Mets general manager, Sandy Alderson married common sense to dollars and cents and traded Francisco Rodriguez to the Brewers.

Though the Mets are still, at the least, on the periphery of the wild-card race, Alderson made this deal for the simplest of reasons: It had to be done.

K-Rod’s $17.5 million vesting option for 2012 was the time bomb ticking on the Mets all season. They simply could not let it explode on next year’s payroll, dimming — among other things — their chances to keep Jose Reyes. So Alderson defused the problem by moving Rodriguez and $5 million to the Brewers for two players to be named.

Look, Alderson was hired because the Mets needed an adult in the room. They needed someone who made reasonable, big-picture choices rather than continue with the decision-of-the-moment way in which the Mets had operated for so long.

This is a move that hurts a little now as the Mets yearn to capitalize on surprising success, returning fan enthusiasm and a Citi Field-heavy second-half schedule. But the reality is that with all the good vibes, the Mets are just a game over .500. They are fourth in the wild-card standings, 71⁄2 games behind the Braves. What are the chances they will play eight games better than Atlanta over the second half?

Five percent? That is not a good enough reason to take a $17.5 million gamble that can begin to destroy their chances for next season — and beyond.

Rodriguez’s contract vests if he finishes 55 games and he was well ahead of that pace with 34. Both Fred Wilpon and Alderson have been public that the Mets’ 2012 payroll is going to be considerably lighter than the $140 million-plus of this year. The general consensus in the sport is that it will come in between $100 million and $120 million.

If Rodriguez were to vest on the Mets’ watch, then he instantly would replace the $18 million that was coming off the books with Carlos Beltran. That couldn’t be justified. Not when the Mets do want to make a concerted effort to keep Reyes, which will mean figuring a way to shoehorn another large contract onto a ledger that already will have Johan Santana, David Wright and Jason Bay at roughly $57 million combined.

The Brewers, conversely, are tied for first place in the NL Central and have John Axford closing successfully. That allows them to keep K-Rod out of the ninth inning and almost certainly prevent him from getting to 55 games finished.

In the offseason, Milwaukee traded what was seen as the last of its top prospects to acquire Shaun Marcum and Zack Greinke. All-Star MVP Prince Fielder is in his walk year and will certainly leave. So this was a season in which Milwaukee has moved its chips to the middle of the pot — the Brewers are all in. So they will take the slight risk of Rodriguez vesting and/or being unhappy in the eighth inning; just a day earlier his new agent, Scott Boras, had said it was preposterous to deal Rodriguez to be a setup man.

The Mets are not in the same situation as the Brewers. They have a new front office trying to figure out how to make the team a substantial long-term contender. And now — with K-Rod gone — they will get to experiment to see if Bobby Parnell or perhaps even Pedro Beato can be an inexpensive closer moving forward.

Though some Mets fans might see this as a betrayal of the team’s playoff chances, especially because David Wright, Ike Davis and perhaps even Santana could be returning in the second half. But those fans should see this as bold, wise and mature; and brace for Beltran to be traded now, as well. This is the anti-Kazmir trade. This is not falling for fake contention and messing up your near future to try to con your fans.

This was Alderson being the adult in the room. This was understanding a few saves in the second half was not as important as saving the 2012 payroll from disaster. Alderson did not let the time bomb blow up on the Mets.

joel.sherman@nypost.com