Johnson told no one, slipped the card into his desk and called it a night. When he returned on Thursday, he looked at the lineup card again.
"It still looked good," Johnson said.
So after four games of hitting seventh, after hitting .385, after making a couple of dazzling defensive plays, Harper did a double take when he passed the lineup card Thursday afternoon.
"It's just another spot in the lineup," Harper said.
Actually, it's not. It's the place where managers typically put their best hitter. It's one of those spots in the lineup that comes with a burden of responsibility.
Some players absolutely hate hitting third because of what it symbolizes. For Harper, it appears to be one more twist in an interesting first week in the Major Leagues.
Have we mentioned that Harper is all of 19 years old? Johnson would like you to stop it.
When the Nats manager moved Harper from seventh to third in Washington's batting order, he weighed only the quality of Harper's at-bats and his seeming comfort level at the things that have been thrown at him.
From the moment Harper was summoned to the big leagues last weekend, the Nationals have cautioned against expecting too much. His agent, Scott Boras, all but predicted that Harper might still find himself back in the Minor Leagues at some point this season.
Isn't there always one to spoil the fun?
Harper stepped into the batter's box in the first inning of his fifth Major League game hitting .385. And now he has gotten the attention of others.
In that first at-bat, D-backs starter Ian Kennedy threw Harper a full-count changeup. Harper is going to see plenty of those in the weeks ahead as scouting reports of him slapping fastballs off the outfield walls get around.
Harper hit that changeup back up the middle, and ended up at second when Kennedy threw the ball down the right-field line.
He wasn't at second long. The D-backs argued that Harper didn't touch first base, and umpire Chris Guiccione agreed and called him out.
Wait, it gets better.
Harper came up in the bottom of the sixth of a tie game with the go-ahead run on third base and one out. The D-backs huddled on the mound, this guy and that guy exchanging notes, ideas, whatever.
"The kid's just 19," Johnson said. "Give him a break."
We're probably past that. Johnson will have another decision to make when third baseman Ryan Zimmerman returns from the disabled list next week.
Zimmerman was Johnson's No. 3 hitter before getting hurt and probably will be again. Johnson has also been without his cleanup hitter, Michael Morse.
Harper could be slipped into one of those spots, but here's betting he eventually settles in as Johnson's No. 2 hitter.
"He plays really hard more than anything," Kennedy said. "He has really quick hands which is not surprising. That's all you can really ask for out of somebody of his status where he's being crowned the savior."
Harper has dominated the headlines and helped the Nationals win back-to-back games after five straight losses. But there are plenty of other reasons for optimism, and all of them begin with a pitching staff that leads the Major Leagues in ERA.
The Nats, who are 16-9 and in first place in the National League East, have allowed just seven home runs, fewest in the Majors and had a string of nine consecutive quality starts snapped on Wednesday.
"We want to push each other to the highest potential we can," Nationals starter Gio Gonzalez said. "These guys are constantly working. Just being around them is fun and exciting."
That rotation begins with Stephen Strasburg, who was named NL Pitcher of the Month for April after compiling a 1.13 ERA in five starts and averaging 9.6 strikeouts per nine innings.
"The other guys see him going after guys, and they kind of pick up on that," Washington pitching coach Steve McCatty said. "That feeds into them."
So far, so good.
"What you're going to get from us is we're going to go out and play as hard as we can every day," Nats shortstop Ian Desmond said. "At the end of 162 games, we'll see where we're at. We feel good. We feel we've got a team that can compete with 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.