The Saints expect him close to 100% for the start of the playoffs, sources told ESPN's Adam Schefter on Saturday.
By going on IR, Thomas must miss at least three games. He had been playing through the ankle injury since returning in Week 9.
Thomas' ankle just was not getting better with practice and games, sources told ESPN. He did not practice at all this week before being ruled out for Sunday's marquee showdown against the Kansas City Chiefs in the Mercedes-Benz Superdome.
The timing is unfortunate for a Saints team that just formally activated quarterback Drew Brees from his own four-game stint on the IR because of 11 broken ribs and a punctured lung. That dynamic offensive duo has only played in two full games together this season.
The Saints (10-3) are currently slotted into the NFC's No. 2 seed because they would lose a head-to-head tiebreaker to the Green Bay Packers (10-3).
Thomas, who was named the NFL's Offensive Player of the Year in 2019 with a league-record 149 catches, has only played in seven games this season. He originally suffered a high ankle sprain in the final minutes of a Week 1 victory against Tampa Bay.
When he was close to returning to the lineup, Thomas was benched for one game as discipline for a practice altercation that included him punching teammate C.J. Gardner-Johnson. Then he suffered a hamstring injury that sidelined him for two more weeks.
Thomas has been productive since returning from the injuries, however. Over the past six games -- four of them with backup QB Taysom Hill -- Thomas has 37 catches for 421 yards, but no touchdowns.
"I'm proud of him because I know it's been a grind working back," Saints coach Sean Payton said last week. "He's still trying to get to 100%. Obviously the injury he had was significant. But, man, he's come up with some big plays here. And for a quarterback new in the lineup, to have that target and someone who's gonna come down with the ball, make plays, that's a big deal."
The Saints made a flurry of roster moves Saturday afternoon, in addition to placing Thomas on IR. They placed receiver-kick returner Deonte Harris on IR with his lingering neck injury. Harris, who was a first-team All-Pro last year as an undrafted rookie, has been sidelined since Week 11.
The Saints also signed wide receiver-kick returner Tommylee Lewis, receiver Juwan Johnson and guard/center Will Clapp to their active roster from their practice squad. They elevated receiver Lil'Jordan Humphrey to the active roster from the practice squad for Sunday's game. And they waived tight end Garrett Griffin and quarterback Trevor Siemian -- who was signed as extra depth while Brees was out.