LeBlanc saw a lineup that included veterans Harper, Werth, Ryan Zimmerman and Adam LaRoche.
"That's what I'm going to face during the season so I'm glad Washington brought those guys over," LeBlanc said. "I could see what I need to work on, and see what works and then try to improve on the rest of the stuff."
Washington's Kurt Suzuki had two hits, including a solo homer off Tom Koehler in the ninth. Werth had two hits and a sacrifice fly.
Giancarlo Stanton's RBI double and Rob Brantly's two-run double highlighted Miami's four-run sixth off Zach Duke.
NOTES: The Nationals acquired LHP Ian Krol from Oakland, completing a three-trade in January that sent OF Michael Morse to Seattle. The 21-year-old Krol was a combined 2-9 with a 5.20 ERA as a starter and reliever between Double-A and Single-A last year. He was Oakland's minor league pitcher of the year in 2010. ... Johnson said RHP Rafael Soriano has been bothered by a wisdom tooth, which could require surgery in the near future. He is scheduled to pitch next on Friday. ... Marlins 2B Donovan Solano (stiff back) was a late scratch. He will be evaluated Friday following Thursday's off-day.