Johnson, who suffered the injury to his left hand in last week's loss to Mississippi State, could miss the remainder of the season, sources said.
The junior has thrown for 517 yards and three touchdowns for the Aggies, who'll take on No. 1 Alabama on Saturday. He was 18-of-25 passing for 201 yards and a touchdown against Mississippi State before leaving in the fourth quarter. He was also sacked four times.
King replaced Johnson in Saturday's 42-24 loss to Mississippi State, going 6-of-13 for 49 yards with two interceptions.