Lopez was officially recalled to start Tuesday's game against the Dodgers.
The move was announced Sunday, but the Nats opted to wait until Tuesday afternoon to officially add Lopez to the Major League roster. The 22-year-old has fared well at both Double and Triple-A this season and Tuesday will mark his MLB debut. It's unclear how long he'll remain with the Nationals, though, as Joe Ross (shoulder) is expected back within the next two or three weeks.