The Pirates’ trade of Proven Closer Mark Melancon prior to last season’s trade deadline was met with much hostility at the confluence of the Allegheny, Monongahela, and Ohio Rivers. The trade was seen in the public, and some corners of the clubhouse, as a white flag being raised, the Jolly Roger lowered at PNC Park. The Pirates remained on the fringe of the postseason picture at the time of the deal. Melancon had been a fixture of the 2013-15 playoff teams.

But those who aren’t focused solely on the Stanley Cup finals are no longer complaining in Pittsburgh. In return for Melancon, the Pirates acquired pitching prospect Taylor Hearn and a headline piece in Felipe Rivero, who is becoming one of the game’s elite left-handed relief pitchers.

We know all about the talents of Zach Britton, Aroldis Chapman, Andrew Miller, the sport’s most dominant left-handed relievers. But Rivero is on the brink of joining their company. And if you’re interested in who’s occupying the ninth inning for major-league clubs — say, fantasy baseball purposes — Rivero might soon be closing games for the Pirates due to Tony Watson’s struggles.

Oh, how the Washington Nationals would like to have Rivero and his four-plus seasons of control back. As we inch closer to the trade deadline, the Rivero deal serves both as a model and warning in how to operate at the trade deadline, where overpaying is almost always a mistake and heeding the lessons of the Stanford marshmallow experiment is typically wise.

To be fair, the Nationals didn’t think they were trading this kind of arm away for 60-plus days of control over Melancon. Like many pitchers before him — A.J. Burnett, Jason Grilli, Francisco Liriano, Ivan Nova, Edinson Volquez, and, yes, Melancon himself — Rivero has made significant improvements since being traded to Pittsburgh.

Let me offer a quick a synopsis on the Rivero story: the lefty has refined his command since arriving in Pittsburgh, he’s added velocity — hitting 102 mph this season — and, in addition to a quality slider, he now boasts an elite changeup.

He’s trimmed his walk percentage from 10.1% with the Nationals and Pirates last year to 5.2% this year.

Met’s play-by-play man Gary Cohen called the fastest pitch of Rivero’s career “Chapman like” earlier this season during an at-bat when poor Michael Conforto saw five 100-plus fastballs:

And that is 102 located at the knees. Rivero’s fastball ranks sixth in average velocity (98.4 mph) among relievers (at least 200 thrown).

His slider has been a ground-ball tool, with a 7-to-1 GB:FB ratio,a figure which ranks seventh among relievers (at least 50 thrown) and is fueling his 63% ground-ball rate, though it can also be a strikeout weapon:

He’s also getting plenty of miss in and out of the zone. His changeup ranks ninth in whiffs/swing (46.55%) among relievers.

More velocity, better control, a ground-ball pitch, and a bat-missing changeup are the ingredients for a dominant WPA-boosting arm in whatever role he is used.

Let’s consider this sequence against Manny Machado.

Good morning (changeup):

Good afternoon (changeup):

Good night (elevated 100 mph fastball):

The Nationals are probably wondering where that changeup came from, since he rarely threw it during his time in the organization. Dave Sheinin investigated the pitch for the Washington Post.

“Never,” Pirates pitching coach Ray Searage said when asked whether he’d ever seen a better change-up. “Yeah, never. The arm action looks like a fastball, and then it comes in about 10 miles per hour slower than the fastball, with movement. It’s amazing. He’s just blessed.”

What’s encouraging for the Pirates is Rivero can get misses out of the zone and is also effective in the zone.

As Jeff Sullivan noted in February, Rivero had an excellent combination of pitching in the zone and limiting contact. Rivero owns the 17th-highest zone percentage (53.8%) among relievers, and the 40th-lowest zone-contact percentage. That is indeed a good combination.

While some have been curious to see what Chapman or Miller could do in a major-league rotation — could either be something close to reincarnation of Randy Johnson? — Rivera actually has more of a starter kit, with three plus pitches and excellent command.

While Rivero has been great — as his 0.90 ERA and 2.59 FIP indicate — there,s also room to grow. He could increase his 34th ranked K-BB (22.8%) by less often throwing his fastball (56.7%) and more often throwing his changeup (23.7%). And Rivero has increased his changeup usage from 5.0% in 2015, to 21.2% last season to 23.7% this season.

He has the skill set of a starter, and he’s also mixing pitchers like most starters. But perhaps he should begin to lean on his changeup like Miller leans on his slider, like Britton leans on his sinker.

Rivero is already very good, with room to grow. The only debate in Pittsburgh now is if Rivero should pitch the ninth or to be used in high-leverage situations whenever they occur, like Miller is being deployed in Cleveland. And it is Rivero who has the skill set to become the Miller of the National League.