The Bucks said Ridnour suffered a fracture of his right thumb during practice Thursday and he will miss about four weeks because of the injury.
Ridnour had appeared in 49 games this season and averaged 10.7 points a game while leading Milwaukee with 5.7 assists and 1.4 steals a game.
The Bucks are already without swingman Michael Redd for the rest of the season because of a knee injury while center Andrew Bogut is expected to miss eight weeks with a back problem.
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