Khris Middleton suffered a hyperextended left knee in Monday's 117-103 loss to the Boston Celtics which the Milwaukee Bucks are praying is not a serious injury.

Middleton landed awkwardly upon contact with team-mate Rodney Hood attempting to rebound late in the third quarter, before limping off the court moments later after trying to play on.

Bucks head coach Mike Budenholzer said they would wait until Tuesday to make a determination on the severity of the injury to the 2020-21 NBA champion.

"I think we're terming it a left knee hyper-extension," Budenholzer said at the post-game news conference.

"We'll know more tomorrow but I think there's some hope it's not serious, but you've got to wait and give it time."

Middleton had four points with eight rebounds and three assists on the night, succumbing to injury after 24 minutes on court.

The 30-year-old small forward is averaging 18.6 points, 5.6 rebounds and 5.0 assists per game this season. Middleton averaged 20.4 points in the Bucks' title-winning season.

The defeat to the Celtics leaves the reigning champions with an 18-11 record and third spot in the Eastern Conference.

Giannis Antetokounmpo scored 41 points on Friday and had a triple-double on Sunday but managed only 20 points with eight rebounds and three assists.

"I think we've got to space around him a little bit better," Budenholzer said. "Move him a little bit more. It was a tough night for us, sometimes that happens."

Budenholzer also refused to blame the Bucks' recent heavy schedule with three games in four days.

"Credit to Boston, they played well tonight," he said. "I think we weren’t at our best. Whatever the reasons are, we've got to be better."