Lopez will start Tuesday's game against the Dodgers, Chelsea Janes of the Washington Post reports.
One of the Nationals' better pitching prospects, the 22-year-old Lopez struck out 100 batters in 76.2 innings for Double-A Harrisburg earlier this season, eventually earning a recent promotion to Triple-A Syracuse for his past two starts. He'll make his MLB debut Tuesday against Los Angeles, but his stint in the majors probably won't last long, as Joe Ross (shoulder) should return within the next 2-to-3 weeks, and top prospect Lucas Giolito is also an option in the meantime.