McGrady hurt his back in practice Friday when he landed awkwardly after a baseline layup, trainer Keith Jones said. The injury is a recurrence of a back problem that plagued McGrady when he played in Orlando, Jones said.
Jones said McGrady went to the hospital on Friday night and tests showed the injury wasn't as serious as it's been in the past.
"All that structural stuff was good," Jones said. "This is more of a compression injury and basically a bruise around the spine -- not the spinal cord, but the spine itself."
McGrady did not attend the Rockets' game against New Orleans on Saturday night. Jones said McGrady was already exercising and could be back sooner than three weeks.
"But I'm not betting on that," Jones said. "We'll get him up and get him going as soon as his range of motion allows him to. We'll know more in four or five days."
David Wesley replaced McGrady in the Rockets' starting lineup against the Hornets.
"Every team relies on greatness down the stretch," Van Gundy said. "However, I do believe that good teams can function efficiently with or without one player. Our chore to function without the one player is going to be tested."
The back spasms are the latest in a growing list of injuries that have slowed McGrady in the past few weeks.
The five-time All-Star missed Houston's last two preseason games because of tendinitis in both knees. He bruised his left knee in a collision with Yao Ming in the practice before Wednesday's opener against Sacramento.
McGrady scored 23 points in the opener, playing with a tight wrap on his left leg.
Jon Barry, who led the Rockets with 24 in the opener, said the loss of McGrady will be "a great test" for a team envisioning a deep playoff run.
"Obviously, you hate to lose your best player, but fortunately it's early in the year," Barry said. "We need to get used to it. It will be a tremendous challenge for all of us."
McGrady was sixth in the NBA in scoring last season with 25.7 points per game. The Rockets were 2-2 in four games without him last season, but Van Gundy is optimistic the team can win with him sidelined.
"We were expecting him to play big minutes, score, be a primary playmaker, defensive stopper, defensive rebounder, backup point guard," Van Gundy said. "But that being said, I have total belief we have more than enough to win with."