Haren went a spring-high four innings, allowing two earned runs in a 2-1 loss to the Detroit Tigers on Sunday.
Rick Porcello started for the Tigers and pitched five shutout innings, allowing three hits while striking out four.
Haren signed as a free agent after going 12-13 with a 4.33 ERA for the Los Angeles Angels last season. He is set to join Stephen Strasburg, Jordan Zimmermann, Gio Gonzalez and Ross Detwiler in the rotation.
Haren says he already has the pecking order figured out.
"Let's see, I am the oldest starting pitcher and I am by far the slowest," Haren said. "I am the second-best hitting pitcher. Other than that, we are a staff and a team that is built for the long run. Our fans in Washington have a lot to be excited about for a long time."
Haren has 119 career wins and has been an All-Star three times. He said he has his eyes fixed on bigger things this year as part of a team that many are picking as the favorite to go to the World Series.
"It's an exciting time to be here in Washington," Haren said.
Nationals manager Davey Johnson has been impressed by what he's seen.
"Dan Haren has been great," Johnson said. "He's been sharp all spring and he has great command. I like what I am seeing."
Smyly hasn't allowed a run in nine innings over three starts. Porcello has only allowed three runs in 12 innings in four starts.
Porcello has fanned 14 without a walk this spring, and has a 2.08 ERA. His name has been mentioned in trade speculation, and he said he's done talking about possible deals.
"I feel good right now," Porcello said. "Everything is working well. I am trying to get ahead of the batters, but I am pitching with confidence. I am not here to work on stuff, I am here to win."
"Numbers are nice, but you can't take too much out of them," Porcello said. "For five innings I felt great today and I need to bring that to the regular season."
Tigers manager Jim Leyland isn't showing his hand on the final opening in the Tigers rotation. He liked what he saw from Porcello.
"That was impressive. He's bearing up under all the talk like a professional. He is locked in now," he said.
Andy Dirks had two hits for the Tigers, improving his spring average to .308. He is in competition with Brennan Boesch and Quintin Berry for the left field slot, and all three played against Washington.
Washington ace Stephen Strasburg will face Atlanta's Paul Maholm on Monday in Viera, Fla. ... Nationals reliever Drew Storen pitched his fourth straight scoreless inning. ... Tigers ace Justin Verlander will make his fourth start of the spring on Monday against the New York Mets.