A stress injury means Hardaway has either a stress reaction or a stress fracture, with the latter being more serious.
Based on information received from medical experts, the New York Post reported that a stress reaction could sidelined a player for two to six weeks, while a stress fracture could keep a player out for two months.
Hardaway has already missed two games because of the injury and he has begun the rehabilitation process.
The Knicks say Hardaway will be re-evaluated in two weeks.
The Knicks have lost five of their past six games, and losing Hardaway is a major blow. He was having his best professional season, averaging 17.8 points, 4.2 rebounds and 3.3 assists per game.
"He's been huge," Knicks coach Jeff Hornacek told the Post. "Obviously Tim has taken that leadership role but Tim's right up there also on the court. ... He's got the ability to be outside and can also drive the ball. That 3-point threat makes guys get up on him and allows him to drive. He can get a shot off anytime he wants. Not a lot of our guys can."
Rookie Damyean Dotson had been starting in place of Hardaway the past two games.
In one bit of good news for the Knicks, forward Kristaps Porzingis, the team's top scorer at 25.8 points per game, might be back for Wednesday's game against the Memphis Grizzlies after missing two games with a sprained ankle.
Porzingis is listed as questionable for that game.