Lillard is expected to be unavailable for Thursday's game against the Houston Rockets and is considered day-to-day, a league source told ESPN's Chris Haynes.
Lillard rolled the ankle in the fourth quarter of Portland's 115-109 loss to the Mavericks. He played the final five minutes, but the pain worsened afterward and he was seen limping in the locker room.
"I think I have a pretty high pain tolerance, but I'm going to be smart about it,'' Lillard told NBC Sports Northwest after the game. "It's the end of the season, we are going into the playoffs, so I have to be smart that one thing doesn't turn into another.''
Lillard said he thought he landed on somebody's foot, but wasn't sure who.
"It's sore, man,'' Lillard said. "I was just in a vulnerable position. ... It's painful. I was able to finish the game, but it was worse when the game was over. Dealing with a lot more pain and swelling.''
Lillard is averaging 26.7 points, 6.6 assists and 4.5 rebounds per game for the Blazers, who have clinched a playoff berth and are three games ahead of the Spurs and Jazz for the No. 3 seed in the Western Conference with four games remaining in the regular season.
The Associated Press contributed to this report.