LAS VEGAS -- Boston Celtics rookie Robert Williams has an artery condition in both of his legs, multiple sources tell MassLive.

A source described the condition as, "not too serious." It could, however, require a procedure if it were to degenerate down the line.

Popliteal artery entrapment syndrome (PAES), the vascular disease Williams has been dealing with, is found most often in athletes. Because of where the muscles and tendons around Williams' knee are positioned, they end up compressing the "popliteal artery," the main one behind the knee, restricting blood flow to the lower leg. It can lead to cramping and calf pain during exercise, per Johns Hopkins Medicine.

NBA teams, including the Celtics, were aware of Williams' condition at the time of the draft -- as was Texas A&M, where Williams went to college. The rookie has been playing with the condition for years, sources told MassLive. The L.A. Clippers' doctors performed his predraft physical and disseminated results to other organizations upon request, standard procedure for a prospect. The Celtics have also done their own physicals on Williams.

The team will regularly monitor his PAES. There are no signs at this time of it getting any worse than it's previously been, per multiple sources.

Williams has been dealing with tendinitis in his left knee since last year, he told reporters after the Celtics' Monday win at Las Vegas Summer League. He has missed two of the team's three games in Vegas after suffering a left knee contusion during the first quarter of the summer league opener.

The Celtics drafted Williams with the No. 27-overall pick in this past June's NBA Draft, though experts expected him to go higher. Many within the league consider Williams a top-15 talent within his draft class. Questions about motivation pushed him further down teams' boards.

He averaged 10.4 points, 9.2 rebounds and 2.6 blocks at Texas A&M this past season, his sophomore year.

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Fred Katz covers the Celtics for MassLive.com. Follow him on Twitter: @FredKatz.