The Post Sports Live crew debates whether Bryce Harper's comments about manager Matt Williams' lineup are potentially damaging for a team that just got back to full strength on Monday. (Post Sports Live/The Washington Post)

Just 14 months ago his numbers were so gaudy, his home runs so prodigious, his candor so refreshing, that the benchmark for Bryce Harper became the surreal rookie seasons of Mel Ott, Mickey Mantle, Ty Cobb and Ken Griffey Jr. His swing mirrored the Sultan of Swat and just about all of baseball was enraptured by his blend of passion and power. He was 20 going on immortality.

Man, what a difference a little more than a year makes.

Today, Bryce Harper is 21 apparently going on 12.

After missing 59 games because of a surgically repaired thumb, Harper decided he wasn’t happy before Monday night’s return against the Rockies.

He wanted to bat higher than sixth in the order, he said, disappointed he found out via social media. He also wanted to play center field instead of left field, where he believed Ryan Zimmerman should play.

Mostly, he decided he wanted to play Matt Williams.

He second-guessed most of his manager’s lineup card and probably alienated Denard Span, whom Harper would have benched. He did this because every 21-year-old major leaguer who has missed 100 games because of injuries over the past two seasons and has already been disciplined for not running hard for 90 feet feels entitled to vent his feelings publicly.

Actually, they don’t.

The worry grows in Washington that one of its prodigal-son athletes and his direct supervisor might have irreconcilable differences — though Williams said before Tuesday’s game, “I got Bryce’s back in every way. He and I are good,” — and if the Big Marine can’t whip Private Swagger into shape, into playing and respecting the game the right way, who can?

I think the concern is larger for Harper. He is quickly using up every drop of goodwill from a once-bottomless reservoir. He is not letting down merely the Nationals; he is bringing into question everything baseball believed he was: that rare phenom who could handle it all — major league sliders, a 162-game grind and the celebrity that comes with being that young and that good.

Monday night’s stunt is just the latest in a series of miscues. In April, Williams benched him for dogging it to first base. Even his one-time mentor, Jayson Werth, is privately rolling his eyes, wondering when this kid is going to thoroughly understand day-to-day professionalism.

Harper is not a bad guy, in the literal sense of a player whose surly attitude negatively affects an entire clubhouse. But he’s like that annoying little brother who is so unpredictable you simply can’t trust him to subjugate himself to the good of the group for any prolonged period of time.

Memo to the hellion trying to stretch another slap single into a triple, obliterating every boundary Yasiel Puig has yet to: It’s not just the game waiting for you to grow up anymore; your teammates are waiting, too.

Many of them want to know when all the glossy magazine covers and Bryce Hustle narratives are going to amount to anything more than a stunted media campaign.

Thirteen months ago, Fox Sports had a headline entitled, “Trout vs. Harper — Who Ya Got?” Brought up to the majors on the same day (after Trout had played some in 2011), named rookies of the year for their respective leagues on the same day, Harper and Mike Trout’s friendly bicoastal battle for numbers and respect had all the makings of an arms race to see who would first become baseball’s next transcendent position player.

A year later, because of Harper’s insolence and injuries and mostly Trout’s ascension into orbit, there is no debate.

This is nothing to be ashamed of, and by all accounts Harper is really happy for Trout’s continued excellence. Not everyone who ends up on the cover of Sports Illustrated at 16 has to remain a prodigy.

But it’s fair to ask what’s going on when we’re talking about a guy who became the youngest player to ever hit two home runs on opening day; a guy with a cannon of an arm from the power alleys; a guy of whom Nomar Garciaparra said, “The reason a lot of baseball people say, ‘This guy plays the game right,’ is the way he hustles all the time;” a guy of whom Al Kaline said, “I was good at 20, but not as good as he is;” a guy whom baseball’s past and present universally agreed represented what is right with the game.

Maybe it’s partly our fault. Blinded by the hype, did we run right through the sign to hold at third and not make enough of his earlier transgressions, from the crotch-grab in Game 5 against the Cardinals to the anger-management moments that were brushed off as competitive fire?

I don’t know. Like with Robert Griffin III or any other young star painfully maturing on the fly, the truth is probably somewhere in the middle. We embellish their bone-headed mistakes as much as we do their athletic magnificence.

Beneath the facial hair, the gruff in his speech after games and his role as a ubiquitous poster child selling tickets for a franchise, he really is still just 21, and so much of his story and progression as a player and a person has yet to take place.

Still, just for a homestand or two, Bryce Harper acting the part of a young big leaguer instead of an aspiring prima donna would be really nice.

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