Stewart is a three-time NASCAR Sprint Cup Series champion and was in contention for a fourth, but he'll miss the next Cup race Sunday in Watkins Glen, N.Y.
A replacement for Stewart at Watkins Glen -- where he holds the record for most Cup wins with five -- has not yet been determined, nor was it clear how long Stewart might be out of his No. 14 Chevrolet, his team said.
[Update 1:12 p.m., Tuesday: Stewart-Haas later said veteran driver
Stewart, who's known for routinely racing sprint cars at small tracks across the country when he's not in a NASCAR race, was competing in a race at Southern Iowa Speedway in Oskaloosa when the accident occurred.
Nicknamed "Smoke," the 42-year-old Stewart was taken to a local hospital, where he underwent surgery for a broken right tibia and fibula, his team said.
(Sprint cars are high-powered, open-wheel race cars typically raced on small tracks and are unrelated to NASCAR's Sprint Cup Series stock cars, where the name reflects the series' sponsor.)
Stewart was 11th in the Cup standings and had been in contention to qualify for NASCAR's 10-race Chase for the Cup title playoff that starts in five weeks.
Some of Stewart's fellow Cup drivers reacted on Twitter. "Thinking about my buddy Smoke,"