Lowry did not play in the Game 3 loss Friday, did not practice Saturday and was pessimistic about the ankle improving before Sunday's tipoff at 3:30 p.m. ET.
"I'm probably doubtful, honestly," Lowry said. "I don't think I'll be able to play. ... It's not looking great, but I'm not giving up on it."
Raptors coach Dwane Casey said Sunday that he didn't believe Lowry would be able to play Sunday.
"I don't think he's going to go," Casey said.
The Raptors trail the series 3-0. No team in NBA history has successfully rallied from such a deficit to win.
Lowry got more treatment Saturday, adding that he's not "giving up hope" of being ready to attempt playing in Game 4.
Lowry suffered the sprained ankle Wednesday night in Game 2. While he did not classify it as a "high ankle sprain," Lowry said it's high on his ankle. It is an unusual injury because Lowry turned the foot inward instead of outward, as is typical with sprains. That makes it harder to play through the injury.
Lowry attempted to play Friday, but after warm-ups he determined he could not.
Lowry has an opt-out of his contract at the end of the season, which he said he intends to exercise. That means Sunday is possibly his last game with the Raptors.
"It was close [Friday], but I think I made it worse by trying to work out a few times," Lowry said. "I aggravated it a little bit more trying to out there and trying to do things, turn and jump and run. So it's more sore today."
The Associated Press contributed to this report.