CHICAGO (Reuters) - Chicago Bulls forward Tyrus Thomas broke his left arm during a weight training workout on Friday and is expected to be sidelined for between four to six weeks.

X-rays confirmed Thomas fractured the radius of his left forearm and will undergo surgery on Saturday the Bulls said on their website (http://www.nba.com/bulls).

In four games this season Thomas has averaged eight points, five rebounds and 21 minutes.

(Writing by Steve Keating in Toronto; editing by Greg Stutchbury)