Grant will miss the remainder of the season because of a triceps injury, general manager Mickey Loomis said.
"Charles has played a key role for our team since we drafted him six years ago," Loomis said. "This is obviously a loss for our defense, but injuries are a part of our league and it presents an opportunity for other players to step up and answer the call."
The injury is the latest setback in a turbulent year for the 30-year-old Grant, who was charged in connection with a bar fight in Georgia last winter and more recently has been under investigation by the NFL for allegedly using a banned diuretic.
Despite the off-field distractions, Grant was tied for the team lead in sacks with three through the Saints' first eight games and was fifth on the team in tackles with 33.
Last February, Grant was stabbed in the neck and was among several people charged with involuntary manslaughter in the fatal shooting of a pregnant woman following a fight at a nightclub in southwest Georgia. Grant's attorney has said the NFL player didn't have a gun and never should have been charged in the woman's death.
Grant pleaded not guilty and was permitted to continue playing while the case is pending.
More recently, he was among several Saints, including fellow starting defensive end Will Smith and running back Deuce McAllister, who tested positive for a diuretic that the NFL considers a possible masking agent for steroids. The three players could be subject to four-game suspensions as first-time violators of the league's drug policy.
Moving Grant to injured reserve opened a spot on the roster for Thomas, who started 26 games for New Orleans during the previous two seasons. Thomas tore his right triceps in preseason and was released after reaching an injury settlement with the Saints, making him a free agent.
He has spent the past two months rehabilitating that injury, which required surgery.
In two seasons in New Orleans, the 6-foot, 335-pound Thomas had 107 tackles with 6½ sacks and a fumble recovery.