The 38-year-old Tracy lost control and his car slid nose-first into one of the concrete barriers lining the downtown street course.
The oldest and winningest active driver in the Champ Car World Series was taken to St. Mary's Hospital after complaining of back pain. Tests confirmed a compression fracture to Tracy's first lumbar vertebrae.
"This is obviously a very unfortunate situation; it's really a shame because it pretty much ruins the championship for us this year," Tracy said after returning to the track. "I'm very disappointed for the whole team because we had gotten off to a great start in Vegas (finishing second) and we were pretty strong here. We were building momentum and just hoping to be fast and consistent the whole year."
The Canadian driver said he had no indication of any trouble until the car snapped around when he got back on the throttle after driving through turn one.
"I went head on into the wall and then the car jumped up in the air and then slammed back on the ground," he said. "I could tell something was wrong right away.
"Now I'm just going to go home and rest for a couple of days and then, early next week, we'll start figuring out the best recovery options so I can get back in the car as soon as possible. I feel OK right now after all the medication that the doctors gave me, but I'm just sore and stiff and I just need some rest."
Forsythe Championship Racing said Oriol Servia would replace Tracy in the No. 3 car for at least the rest of the weekend. No timetable was given for Tracy's return.
Servia qualified 14th in the 17-car field for today's race.