The Terps conceded a goal for the first time in 297 days.
Johnston scored the game winner against USF.
The Terps have been outscored 9-0 in their past two games.
The Terps conceded two penalty kicks in the rout.
Coach Ray Leone said a replacement will be decided upon in Georgia.
Midfielder Malcolm Johnston scored the lone goal.
Brown is adjusting to a new school and a new position.
Coach Sasho Cirovski believes another challenging slate could lead to a second straight national championship.
The Terps are ranked No. 1 in the country by the United States Soccer Coaches' preseason poll despite their conference ranking.
The Terps had 44 shots in their first two games, but converted three of them.