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Damian Lillard has been diagnosed with a blood clot in his right calf and will be out indefinitely. Lillard is on blood thinning medication to treat the issue. 

Lillard is averaging 24.9 points, 4.7 rebounds and 7.1 assists in his second with the Milwaukee Bucks. Lillard last played on March 18th when the Bucks lost to the Golden State Warriors.

Via RealGM Staff Report

I'm a doctor. Here's the real medical deal on this:
If the clot is in a superficial vein, the treatment is usually mild and short-term. Occasionally we use blood thinners in that situation but not typically.
But if the clot is in a deep vein, the treatment needs to be at least 3 months of blood thinners.
So if this clot is a DVT (DEEP vein thrombosis), he's not off blood thinners until mid-late June, which means he's almost certainly done for this season.

Doctor Dee wrote:I'm a doctor. Here's the real medical deal on this:
If the clot is in a superficial vein, the treatment is usually mild and short-term. Occasionally we use blood thinners in that situation but not typically.
But if the clot is in a deep vein, the treatment needs to be at least 3 months of blood thinners.
So if this clot is a DVT (DEEP vein thrombosis), he's not off blood thinners until mid-late June, which means he's almost certainly done for this season.

You don't happen to have a brother that's a detective do you?