"It's a tough one for him," Senators coach Guy Boucher said after practice Wednesday. "It's a tough one for us too because he was doing so well. He's a big part of our identity."
The 19-year-old forward, the No. 4 pick in the 2018 NHL Draft, has six points (three goals, three assists) and is plus-5 in four games this season.
It is unclear when the injury occurred; Tkachuk played and finished Ottawa's previous game, a 4-1 win against the Dallas Stars on Monday.
"I knew that there was something there but he finished so well," Boucher said. "If it was up to him, he'd be on the ice today. He's a tough kid and certainly not someone who's looking to bail out that's for sure."
The injury will not require surgery.
"It's a normal tear that has to take its time," Boucher said. "It's not something that you can accelerate. Those types of tears take X amount of days to heal."
The Senators (3-2-1) host the Montreal Canadiens at Canadian Tire Centre on Saturday (7 p.m. ET; TVAS, CITY, NHL.TV).
