An MRI is scheduled for tomorrow in Boulder.
"We'll find out," Colorado coach Tad Boyle told reporters after the game. "I don't know the prognosis right now. It didn't feel good, but right now I don't know."
Boyle said that the 6-foot-6, 200-pound junior, who leads the 15th-ranked Buffaloes in both scoring (15.2) and assists (3.9) per game, will have his left knee examined after returning to Boulder.
"My gut says it's not good, but we'll see," Boyle added. "If he's out, he's out. I feel so bad for the kid because he's worked so hard, but we'll have to wait and see."
Dinwiddie went down late in the first half as his left knee buckled as he dribbled down the floor in the first half. He had seven points at the time of the injury and was helped off the court and into the locker room.
Colorado lost to Washington, 71-54, and fell to 14-3 overall.
The Buffaloes are considered a potential Sweet 16 team and might even be the biggest threat to Arizona in the Pac-12 with the versatile Dinwiddie in the fold. However, if he is done for the season, the Buffs could wind up being a fringe NCAA tournament team.
Dinwiddie is considered a potential first-round pick in June's NBA draft if the California native elects to leave school early.
ESPN.com's Andy Katz contributed to this report.