Karlsson suffered the injury when he blocked a shot during a game against the Philadelphia Flyers on March 28; he missed three of the last five games of the regular season.
He aggravated the injury when he played back-to-back games against the Detroit Red Wings on April 3 and 4. He has been receiving injections in his heel.
"I've been playing on shots here," said Karlsson, who admitted he was having trouble moving to his left. "It's frustrating because you've worked so hard all year, but it's better now and by Thursday it should be pretty much back to normal," he said.
Karlsson led the Senators to a first-round victory while averaging 30 minutes per game in the playoffs, including 41:51 of ice time in a double-overtime loss to the Bruins in Game 5.
The Senators play Game 1 of their Eastern Conference semifinal series against the New York Rangers on Thursday night in Ottawa.
"He's a machine," said Senators coach Guy Boucher. Karlsson wanted to play the final few games of the season for a chance to catch San Jose Sharks defenseman Brent Burns, who led all defensemen with 76 points in 82 games. Karlsson finished with 71 points in 77 games, but felt it was best to rest his heel before the postseason began.