Milwaukee Bucks star Damian Lillard will miss the start of the NBA playoffs because of a blood clot in his right calf, sources told ESPN.

The Bucks play Game 1 of their first-round series Saturday against the Indiana Pacers.

Doctors examining Lillard, who has been sidelined since March 18, believe the veteran guard is making significant progress with the blood clot shrinking in the calf, sources said. Lillard has yet to be cleared to resume full basketball activities.

The Bucks announced Tuesday that Lillard had made "significant improvement on his deep vein thrombosis" and that the nine-time All-Star will increase his on-court activity. The team did not offer a clear timetable for Lillard's return.

"Damian's most recent weekly scan shows that his injury has significantly improved which will enable him to move ahead safely with increased basketball activity," Bucks general manager Jon Horst said in a statement. "Damian's health remains our No. 1 priority. We have followed strict protocols and will continue to do so. We are pleased with the positive news about Damian's progress."

The Bucks announced last month that Lillard, 34, would be sidelined indefinitely. The team said at the time that Lillard had been using a blood-thinning medication to stabilize the clot, with Horst adding that the Bucks would "support him as he moves through this weekly process of strict criteria to ensure that it is safe for him to return to play."

Bucks coach Doc Rivers said earlier this month that he was optimistic Lillard would return at some point, but acknowledged that the team also was prepared for the nine-time All-Star to miss the rest of the season.

"There's no guarantee we are going to have him," Rivers said on April 3. "We get him back, we know how to play. But we don't know how to play in the long term without him. ... So we got work to do."

Lillard was averaging 24.9 points and 7.1 assists before being sidelined for Milwaukee's final 14 games of the regular season. The Bucks went 10-4 in those contests and ended the season on an eight-game winning streak.

ESPN's Tim Bontemps contributed to this report.