Milwaukee Bucks star Damian Lillard exited in the first quarter of Sunday night's 129-103 Game 4 loss to the Indiana Pacers because of a left lower leg injury.

The preliminary examination of Lillard indicated a possible Achilles tendon injury, a person with knowledge of the situation told The Associated Press.

Lillard is scheduled to undergo an MRI on Monday, when the Bucks are expected to know the severity of the injury, league sources told ESPN's Shams Charania.

"Just being honest, it's not very promising," Bucks coach Doc Rivers said postgame.

Lillard was injured while trying to help secure an offensive rebound midway through the first quarter. After tapping the ball away from a Pacers player, Lillard fell to the ground and grabbed his left leg, remaining down for the next defensive possession.

He was helped to his feet during the next dead ball and needed assistance leaving the court while putting no weight on his left leg. He was escorted to the locker room, and the team ruled him out for the rest of the game.

Lillard had missed the team's final 14 regular-season games because of deep vein thrombosis in his right leg. He returned in Game 2 of the first-round series against the Pacers after a month out of action in a recovery the team called unprecedented.

However, Lillard had not matched his usual production during this series, averaging 10.5 points and 6.0 assists on 24% shooting (6 of 25) in his first two games. He missed his only two shots in Game 4 before exiting.

"I felt bad for him, the guy tried to come back for his team. ... I just felt bad for him," Rivers said.

Milwaukee trails 3-1 in the series against Indiana.