"We started their throwing program when they came to us. So they were a few weeks behind," Harris said.
Lopez will make his first start Wednesday against the Wilmington Blue Rocks, while Giolito will start Friday against the Frederick Keys.
Giolito is one of the reasons the Nationals' farm system is considered strong. Ranked by MLBPipeline.com as the Nats' top prospect, Giolito, 20, showed he was fully recovered from Tommy John surgery last year while playing for Class A Hagerstown. In 20 starts, he was 10-2 with a 2.20 ERA and 110 strikeouts in 98 innings.
Lopez, 21, is ranked the fourth-best prospect in the organization after going 7-3 with a 1.08 ERA between Class A Short Season Auburn and Hagerstown. He issued only 26 walks in 83 1/3 innings, and his fastball has been clocked as high as 95 mph.
In other prospect news, right-hander Erick Fedde, the Nationals' first-round pick in the 2014 First-Year Player Draft (18th overall), is throwing from the mound and hopes to start pitching in extended spring games in about a month, according to Harris.
After being drafted, Fedde immediately had Tommy John surgery, and he hopes to pitch in Minor League games later this season.