"If the first two guys get on," Harper said, "I'm going to drop a bomb and walk off this field and tell 'em, 'We own this place.'"
Nine months later, Harper smiled when asked about that moment late in a game in the Arizona Fall League.
"I was just kidding," he said, "but I was serious at the same time."
Oh, you know how these young ballplayers are with their fish tales.
"Yeah, right," Crawford said at the time.
Harper didn't back down.
"I promise you I'm going to hit a jack right here," he said. "I swear on earth."
Mike Trout laughed, too.
"Yeah, OK," he told Harper.
And this is where the story gets really good.
Trout led off the inning with a single. After a fielder's choice grounder, Will Middlebrooks singled.
Put up or shut up, buddy boy.
"I'm in the on-deck circle laughing," Harper said.
"The pitcher hung a 3-2 changeup," Harper said, "and I hit it out to right-center. Everybody ran in the clubhouse."
Harper laughed and shook his head Monday afternoon as he recounted a moment that spoke volumes on a number of different levels.
One is that all these months later, every player in this story is in the Major Leagues. Crawford is playing shortstop for the Giants, and Middlebrooks has taken over at third for the Red Sox.
And Trout and Harper are two of the record-setting five rookies (alongside pitchers Ryan Cook, Yu Darvish and Wade Miley) in the 83rd All-Star Game tonight.
Actually, they're more than that, way more than that. Trout is 20, Harper 19. Both of them began this season in the Minor Leagues.
But when they got their chance to play, they produced immediately. The Angels are 40-24 since Trout arrived for his second stint in the Major Leagues on April 28, and they have turned around what had been a disappointing season.
Harper was called up the same day for his debut. The Nationals were already in contention when he arrived, and as he said, "I didn't want to do anything to mess up the vibe in the clubhouse."
He helped the Nats weather the loss of their best player, third baseman Ryan Zimmerman, and is a huge part of making this a magical baseball summer in the nation's capital.
Trout and Harper play with energy and passion and seem to be on their way to becoming two of the faces of Major League Baseball. It seems appropriate that both ended up at this All-Star Game.
They'll probably be compared to one another for years, but in the end, they're both good enough to build a franchise around. Trout leads the American League in both hitting (.341) and stolen bases (26) and has played his way into the AL Most Valuable Player Award conversation.
Harper is hitting .282 with eight home runs. Despite being 19 and having just one full season in professional baseball, he has never appeared awed or overwhelmed.
There they were on Monday, at their first All-Star news conferences, handling the media smoothly, telling stories, laughing, having the time of their young lives. Since their Arizona Fall League experience, Trout and Harper (and Middlebrooks and Crawford) have stayed in touch, exchanging text messages, offering support and congratulations.
Now back to that walk-off home run.
"It was pretty amazing," Trout said. "He's gone through a lot. He's been rated the No. 1 prospect since he was 6 years old or something. He was really cool to me."
As for Trout, he came to this All-Star Game with the goal of spending some time with his boyhood idol, Derek Jeter. Trout struck up brief conversations with him at second base earlier this season but intends to pay attention to how Jeter goes about his business.
"I got to meet him a little bit this year, and being on the same team with him is pretty special," Trout said. "I love how he plays the game. I've tried to imitate him. It's incredible to be here with him."
Trout hit .220 during a 40-game stint with the Angels last season. He got hurt in Spring Training and began the season in the Minor Leagues.
Once Trout arrived, he quickly proved he belonged.
"It feels great," he said. "I feel comfortable. I feel like myself. Last year, I didn't feel like myself at all. I was trying to do too much. I was trying to hit that long home run."
"With [Angels manager Mike Scioscia] putting me in the leadoff spot," Trout said. "I'd batted leadoff my whole Minor League career. It's where I felt comfortable. I felt like I was in a good place. The hard work is paying off for me. Hopefully, it carries on in the second half."
They're equally complimentary of one another.
"He plays the game hard," Harper said of Trout. "He plays the right way. He has a winning mentality. ... I'm so excited for him."
Plenty of people around the game are excited for both of them. In a half-season, they've shown us just a glimpse of their greatness. This All-Star Game is the next appropriate step.
"I just tried to go in there and play the game that I knew how to play," Harper said. "I didn't want to come in and change the vibe, because we were winning. I wanted to keep my mouth shut and play the game I know how to play and work hard and respect everybody around me and not worry about anybody."
Richard Justice is a columnist for MLB.com. Read his blog, Justice4U. This story was not subject to the approval of Major League Baseball or its clubs.