A spokesman said that Stewart broke his right tibia and fibula after crashing while leading the 30-lap feature. A lapped car spun in Turn 4, collecting Stewart and two other cars.
SHR has not named a replacement driver at this point, but the team did indicate that they were making preparations as if Stewart would not be available.
Missing Watkins Glen could be devastating to Stewart’s championship hopes. He is in the midst of battling for a Chase berth, sitting 11th in points, in contention for the Wild Card with one victory this season.
Stewart defended sprint car racing earlier this summer after Jason Leffler died in a sprint car accident  at Bridgeport Speedway. At the time he said:
I’d be grateful if you guys would understand that what happened this week wasn’t because somebody didn’t do something right with the racetrack. It was an accident. Just like if you go out and there’s a car crash. It’s an accident. Nobody as a track owner wants to go through what happened, but it’s not due to a lack of effort on their part to try to make their facilities as safe as possible under the conditions they have.
This is not the first time Stewart injured himself during a non-NASCAR race. In 2006, Stewart broke his wrist and bruised his ribs after flipping his midget car in a race. And, Stewart is not the only one.
Jimmie Johnson, Carl Edwards and Denny Hamlin all suffered injuries while participating in activities away from the NASCAR race track. Johnson broke his wrist falling out of a golf cart during a celebrity golf tournament. Edwards broke his foot while playing Frisbee with friends. And Hamlin tore his ACL  playing basketball.
When these things happen, they kick off a debate about whether or not race teams should place restrictions on drivers’ recreational activities.
Update: Veteran driver Max Papis has been tapped to replace Tony Stewart in the No. 14 Stewart Haas Racing Chevy in the Cheez-It 355 at Watkins Glen International. Papis tested that car at Road Atlanta last week.
Stewart underwent surgery to stabilize and clean his broken tibia and fibula. He will undergo a second procedure in the near future.
Update 2: Stewart Haas Racing Competition Director Greg Zipadelli said that there is no timeline for Stewart’s recovery or return to racing.
The team is looking for a substitute driver for at least the next two or three races. They would like to make a decision in the next day or so as to who that driver will be.
He also indicated that SHR would evaluate the races in which Stewart competes, but that doesn’t mean that they will change anything. In other words, it doesn’t seem at this point that Stewart would be told to give up sprint car racing.