Daniel Murphy hits a 1st inning RBI single during last month at Nationals Park. (John McDonnell/The Washington Post)

Last fall, the talk of baseball was Daniel Murphy, a competent career slap hitter who suddenly hit seven postseason home runs, including blasts off Jake Arrieta, Zack Greinke, Clayton Kershaw and Jon Lester. Murphy was probably the top reason the Mets won the pennant, instead of the Dodgers or Cubs.

This year, Murphy is hitting .394 for the Nationals. After a third of this season, he’s on pace for 234 hits, 45 doubles, 27 homers and 102 RBI. This is insanity. Every one of those numbers explodes the evidence of Murphy’s previous 10 seasons as a pro. When the Nats agreed on a three-year, $37.5 million deal with him on Christmas Eve (what a gift), he was a solid .288 hitter with 10-homer power, few walks and just two steals last year. The Nats grabbed him because Ben Zobrist and Brandon Phillips had spurned them.

So how good is Murphy, really? How did he get this way? And how long will it last?

You want answers, not hedges. Here’s mine: Since he and Mets hitting coach Kevin Long radically changed his approach, Murphy has turned himself into a new and far better hitter. He’s probably now a .300 to .320 hitter with 20-to-25-homer power and, because he likes to whack away rather than draw walks, a knack for collecting 90 or more RBI. The Nats, who thought they had bought a nice No. 2 or 6 hitter, probably just got a quality No. 4 or 5 hitter who usually would cost more than $100 million.

Why be so bold? Couldn’t this guy just be hot and lucky? Maybe. The sample size for New Murph is still small enough that he could regress. But that data is no longer as small as many think. Go back further, to the time when his revised hitting approach started to show results: early August of last year.

In his past 115 games and 451 at-bats, including last October’s playoffs, Murphy has hit .335 with 24 homers, 82 RBI and a stunning .970 on-base-plus-slugging average. In that time, he has had good luck. This year, his batting average on balls in play is .411 — totally unsustainable. His career norm is .315 in BABIP. He should be hitting only .356 right now!

However, Murphy’s hitting luck was actually poor in the final two months of last season. This year, he’s just getting paid back — with interest. Put it all together, and New Murph deserves to have hit over .320.

The answer to our second question — How did this happen? — is the most important. Murphy’s new results coincide exactly with a new method. Murphy has set the sport on its ear by having the audacity to completely transform his approach to hitting. Ol’ Daniel’s gone.

The 220-pound left-handed hitter moved much closer to the plate — perhaps as much as half a foot. Before, he almost stood “off the plate.” Now he crowds it, his right elbow almost over the inside corner. Even his stance is slightly closed rather than square. The goal: pull the ball more often. He also went from standing fairly upright in the box to crouching almost half-a-foot lower than he was before. The goal: hit slightly below the center of the ball more often rather than slightly on top. More flies, fewer grounders.

Long made his suggestions before the season, but it took until early August for all the changes to work together. The crouch also allowed Murphy to drive more with his legs while keeping his hands lower and closer to his body. His slightly closed front foot barely rose at all — wait, yet still be quick and pull.

The difference is almost unheard of. In 2012, Murphy had a ratio of 2.03 groundballs for every fly ball; that always puts you among the most extreme groundball batters. (FanGraphs was the first to analyze this.) Last year, although he changed late in the season, his ratio dropped to 1.19. So far this season, it’s 0.64 — making Murphy perhaps the most extreme flyball hitter in the game. A total identity flip!

His tendency toward more pull-hitting is distinct but not as radical. Before this season, Murphy hit 31 percent to left field and 33 percent to right field. This year, it’s 22.5 percent vs. 38.2 percent pulled. And when you pull it in the air, home runs happen.

With all this change, you would expect negative secondary effects. Good luck finding ’em. He doesn’t swing and miss any more than usual, still a superb 5.5 percent. In our 115-game sample, Murphy has 24 homers with just 49 strikeouts. Murphy chases the same percentage of pitches outside the strike zone as always (not many), but he has reduced the number of pitches within the zone that he will swing at: “Come to me.”

What confidence it required to make such changes, especially for a man who hit .281 to .320 every year his old way. Perhaps only a batting-cage-rat and mechanics-theorist such as Murphy would dare to try it. They said Stan Musial could roll out of bed at 3 a.m. and hit. Murphy probably is still taking BP at that hour.

To a pitcher or scout trying to analyze how to pitch to Murphy, this is like a player being taken away at midnight by vampires and returned the next day as an entirely different — and far more dangerous — species. So far, they’re dumbfounded. This year the league has thrown the same percent of fastballs to Murphy as it has in the past while cutting down on sliders (maybe thinking the homers are the product of a “slider-speed bat”) in favor of more change-ups and curves. How’s that working out?

Our third question — how long does it last? — is hardest. You can’t know. A 115-game sample is nice. It has meaning. But a three-season sample will be decisive. Lucky Nats: They got the three seasons. But the burden of proof has shifted. Now Murphy will have to prove that he’s not a No. 3-4-5 hitter.

We have all asked Murphy about his new hitting secrets — the ones you can’t simply see with the eye or spot in stats. He usually says, “Just lookin’ for a good pitch.”

The day Murphy was introduced in D.C., General Manager Mike Rizzo called him “the best left-handed value we could get. And this guy really plays the game the way I like: gritty, hard-nosed but with a smart baseball mind.” Rizzo forgot to mention the .394 batting average.

That average, or anything close to it, won’t endure. But the mighty fine New Murph should remain.