Harper will make his Major League debut in left field on Saturday against the Dodgers at 9 p.m. ET. The game will be available on MLB.TV, and MLB Network will also air the game for out-of-market fans.
At 19 years old, Harper will become the youngest player to suit up for the Nationals. Zimmerman was the youngest, making his debut at 20 years old.
"We can use the left-handed bat in Harper," manager Davey Johnson said. "I'm going to put him in left field and let him play. So he was a natural pick. He has been swinging the bat well."
Washington made the decision to promote Harper after learning that Zimmerman would require time on the DL. The Nationals considered Tyler Moore for the left-field spot, but the team felt that Moore was not yet ready to play the position in the big leagues.
The Nationals also need another productive bat in the lineup, since left fielders Roger Bernadina, Xavier Nady and Mark DeRosa entered Friday hitting a combined .097 with four RBIs. It doesn't help that the team's cleanup hitter, Michael Morse, has yet to play a game this season because of a torn right lat (shoulder) muscle.
"The decision is a team decision," Rizzo said. "It supports the Major League club. We need an offensive player that can play the corner outfield. It was a choice between a couple of players on the farm. Tyler Moore just didn't have the experience to play left field at the Major League level yet. He just started to play there several weeks ago."
The Nationals still see Harper as the center fielder of the future.
"We still have a very good and committed plan for Bryce," Rizzo said. "I still believe very passionately in the plan, and we are going to be very committed to it. But this is [a move made] by the circumstances -- by two of our middle-of-the-lineup guys going down. And we felt we needed to bring in a left-handed bat that can play the corner outfield."
Entering Friday's action, Harper was hitting .250 with a home run and three RBIs for Triple-A Syracuse, but Rizzo said Harper was hitting the ball much better this week. In his past 10 games, Harper hit .290 with a .420 on-base percentage.
Harper was hoping to be on the Opening Day roster, but he was optioned to Syracuse after going 8-for-28 (.286) with a team-leading 11 strikeouts during Grapefruit League play this spring. It didn't help Harper's case that he missed six games because of a calf injury.
At the time, Johnson said the timing wasn't right for Harper to be on the Opening day roster.
"He knows how much I love his talent. He is a special player," Johnson said after Harper was sent to the Minors. "He needs to go down there and do the same thing he is doing. He doesn't need to change anything for me. He just needs to go play.
"I basically said to him, 'If I had you up here and you were 1-for-10, I would probably have people [saying they rushed him].' I don't want that hanging over us."
As for Zimmerman, Rizzo doesn't think he will be on the disabled list for long. The third baseman had inflammation on his AC joint and said the shoulder has bothered him since he made several diving plays last week. Zimmerman had a second cortisone put in his shoulder on Friday. He will start throwing the baseball on Tuesday, and swinging the bat in a week.
This past Saturday, Zimmerman was taking batting practice -- with hitting coach Rick Eckstein observing -- when his shoulder tightened up on him. Zimmerman was scratched 30 minutes before the game started against the Marlins.
"It's disappointing, because you obviously want to play," Zimmerman said. "I think it's good to know that you have some closure and stuff done. If all goes as planned, it's not going to be very long at all. Hopefully, we'll look back on it in September and not even remember it."
With Zimmerman out of the lineup, Steve Lombardozzi will get most of the playing time at third base. Entering Friday's action, Lombardozzi was hitting .364 with three RBIs.