Sources: Bucks preparing to be without Giannis Antetokounmpo

The Milwaukee Bucks are preparing to be without two-time MVP Giannis Antetokounmpo for the start of their Eastern Conference first-round series against the Indiana Pacers but are hopeful treatment on his left calf strain will allow him to return sometime later in the series, sources told ESPN on Tuesday.

Antetokounmpo has been undergoing around-the-clock treatment on the calf, sources said.

"Until we know if he's playing or not, we're going to prepare as if he's not playing," Bucks forward Khris Middleton said after practice Tuesday. "Simple as that."

Antetokounmpo was not the only Milwaukee star that did not practice on Tuesday. Bucks coach Doc Rivers said Damian Lillard sat out Tuesday's practice after dealing with groin and adductor injuries near the end of the season. Rivers said Lillard received an MRI that came back clean.

Lillard said following Sunday's regular-season finale in Orlando that he would likely not be 100% for the rest of the season.

"We feel like -- let's get him a little more rest," Rivers said. "He's still doing conditioning and stuff like that, but we went [5-on-5] a lot today and we didn't want him out there."

"We want him to be as close to 100% as possible if you can be that at this point."

The third-seeded Bucks open the series against the sixth-seeded Pacers on Sunday in Milwaukee. The Pacers won four of five regular-season games against the Bucks.

Antetokounmpo missed the final three regular-season games after crumpling to the floor with a noncontact injury a week ago against the Boston Celtics.

Antetokounmpo ended the regular season averaging 30.4 points on a career-best 61% shooting with 11.5 rebounds and 6.5 assists. He's the first player in NBA history to average 30 points per game on 60% shooting, according to research by ESPN Stats & Information.

With Antetokounmpo's potential absence to start the series, Middleton said the Bucks ran actions in practice that highlighted both him and Lillard, should they have to shoulder the offensive load without him.

"It's a work in progress," Middleton said about the chemistry between him and Lillard. "Trying to figure out how we can both create advantages for ourself and the team. We've been working on that. It's going to be a weeklong process and hopefully we can build on that as we go, and once Giannis gets back."

ESPN's Jamal Collier contributed to this report.