An MRI on Sunday confirmed that Knox suffered a sprained ankle in the first quarter of the Knicks' loss to the Boston Celtics on Saturday, the team said.
Knox will not travel with the Knicks on their two-game road trip this week and will be re-evaluated in seven days. So the rookie will miss at least three games because of the injury.
Knox, 19, was driving for a layup on a fast break Saturday when his ankle appeared to roll badly as he was fouled by Terry Rozier. Knox stayed on the court briefly, appearing in pain. He didn't put any weight on his left leg after the injury and was helped off the floor by teammates.
The No. 9 overall pick in the 2018 NBA draft, Knox has been coming off the bench for New York early in the season. He played well in the Knicks' loss to the Brooklyn Nets on Friday night, scoring 17 points on 7-for-14 shooting.
"I feel bad for the kid," Knicks coach David Fizdale said Saturday night.
If Knox is sidelined for a significant amount of time, it would be a tough loss for New York. The Knicks (1-2) are already without All-Star forward Kristaps Porzingis, who is rehabbing from ACL surgery.