Johnson suffered the injury in Wednesday night's 75-67 loss at Texas A&M when he stepped on an official's foot in the first half. Johnson was re-examined when the team returned to South Carolina.
The 6-foot-3 transfer from Villanova came into the week averaging a team-leading 11.9 points a game. He has played in 16 games, nine of those as a starter.
Johnson is the second point guard the Gamecocks have lost this season. Bruce Ellington, who was also the football team's leading receiver as well as a three-year basketball starter, gave up his senior seasons in both sports to enter the NFL draft.
The Associated Press contributed to this report.