The Wizards said Tuesday that Brown strained his calf a day earlier during what the team called a routine workout.
Washington opens the regular season one month from Monday.
Brown averaged 4.8 points, 2.8 rebounds and 1.5 assists in 52 games as a rookie last season after getting drafted in the first round out of Oregon.
Wizards point guard John Wall is expected to miss most, if not all, of next season because of a ruptured Achilles tendon. Another guard, Isaiah Thomas, had surgery on his left thumb last week and could miss two months.
Forward CJ Miles had surgery for a stress fracture in his left foot in July.