St. Louis announced the agreement with the 37-year-old right-hander on Oct. 11. He has finished a $97.5 million, five-year contract that paid $19.5 million annually.
Under baseball's labor contract, his guaranteed salary could not be reduced by more than 20 percent if he remains on the roster continuously. That means he would have to be guaranteed at least $15.6 million.
His new deal contains a salary less than that figure, people familiar with the agreement told the AP, speaking on condition of anonymity because the deal had not been finalized. The maximum-cut rule would not apply if he becomes a free agent following the World Series, and then agrees to the new deal with the Cardinals.
Wainwright was 2-4 with a 4.46 ERA in eight starts this year, sidelined by hamstring and elbow injuries. He pitched four times in September.
A three-time All-Star, Wainwright is fifth on the Cardinals' career list with 148 wins and second with 1,623 strikeouts. He was the closer for the Cardinals' World Series championship team in 2006.
St. Louis also won a title in 2011, but Wainwright did not pitch that year.