Richardson is averaging 14.1 points, 3.7 rebounds and 3.6 assists a game. He will not travel with the team to Las Vegas for the tournament, Oregon announced Tuesday.
A senior, Richardson did not play in the team's regular-season finale at Washington State on Saturday because of the illness. The Ducks lost 94-74.
Fifth-seeded Oregon plays No. 12 seed Oregon State to open the league tournament Wednesday.