Harper was called up on Saturday, but isn't feeling the pressure that comes with the debut of a No. 1 overall Draft selection.
"I didn't have butterflies at all, really," Harper said after his first Major League game Saturday against the Dodgers. "It was the first time I didn't get butterflies. I was talking to [Adam] LaRoche before the game. I told him, 'Hey, I'm really calm right now.' I wasn't upbeat or anything like that. I was pretty calm. I was trying to look for my pitch and get into some good counts."
Nationals right-hander Jordan Zimmermann had his way with the D-backs last season, and he will try to continue his success against Arizona at home on Tuesday.
In two starts against the D-backs in 2011, Zimmermann threw 13 1/3 innings and allowed three earned runs. The Nationals were just 1-1 in those games, however, thanks in large part to Ian Kennedy shutting out Washington over seven innings in one of Zimmermann's outings.
So far this season, Zimmermann is experiencing similarly low run support despite pitching well. In his first three starts of 2012, Zimmermann only received two runs from his offense.
In his fourth and most recent start, however, the Nationals broke out and put up a seven spot in a win against the Padres.
"I felt good," Zimmermann said after his last start. "I threw a lot of changeups [against the Padres]. I threw quite a few, and there were some good ones in there. I had a really good curveball, but I didn't have much of a slider. Whenever I got into trouble, I mixed in a curveball."
In each of his four outings this year, Zimmermann has allowed just one earned run over a total of 27 innings. The Nats were just swept by the Dodgers in Los Angeles and have lost four straight overall.
Opposing Zimmermann will be D-backs offseason acquisition Trevor Cahill. The 24-year-old, who was with the Athletics his entire career before being traded to Arizona, has never pitched against the Nationals.
Cahill's first two starts in a D-backs uniform were promising. In 13 1/3 combined innings, he allowed two earned runs on six hits.
But in his most recent two outings, Cahill has gotten roughed up by two National League East teams. The Braves and Phillies each tagged him for four runs over a combined 11 innings in two losses.
"I was feeling pretty comfortable, but you make a bad pitch and they'll make you pay every time," Cahill said after his April 20 start against Atlanta. "I was thinking I had to make quality pitches, and I'd leave a ball over the plate instead of relaxing."
Cahill didn't pitch against the Marlins in the D-backs' most recent series, so he will try to fix his NL East division woes against the Nationals.
D-backs: Overbay over Goldschmidt
D-backs first baseman Paul Goldschmidt's struggles at the plate to start the season have led to more consistent playing time for veteran Lyle Overbay.
Goldschmidt is batting .193 with eight RBIs in 57 at-bats, while Overbay is hitting .313 in 32 at-bats.
"You want to show confidence in them and play them, but at the same time, you want to keep everybody playing," manager Kirk Gibson said about balancing playing time. "There's times I think that you want to try to balance it, give a guy some time to relax and work on some things."
Nationals: Moore makes debut
Two days after the Nationals passed over outfielder Tyler Moore in favor of calling up Harper, Moore made his Major League debut Sunday against the Dodgers.
The move came with infielder/outfielder Mark DeRosa being placed on the 15-day disabled list with a left oblique strain.
Moore was hitting .286 with seven home runs and 20 RBIs at Triple-A Syracuse before starting in left field for the Nationals on Sunday in place of Jayson Werth, who was a late scratch. Moore went 1-for-3 with a single in his debut.
Jason Kubel, who sat out Monday's game due to illness, has an 11-game hitting streak, but has never faced Zimmermann.
The D-backs won three of four games in Washington last season, and took five of eight from the Nationals overall in 2011.
Justin Upton and Gerardo Parra went a combined 5-for-10 against Zimmermann last season.