The Nuggets beat the Thunder 127-124 Thursday at the Pepsi Center in Denver, with Gary Harris hitting a game-winning 3-pointer at the final buzzer.
After the shot went in, fans celebrated and one man ran up to confront the 2017 NBA MVP. The man screamed in Westbrook's face before the Thunder guard used his left arm to push him away. Referees, Westbrook's teammates and arena security separated the parties.
"You've gotta be able to protect the players You play the game, you guard the fans. But, they can't come on the floor. It's totally unacceptable," Westbrook told reporters after the loss.
"They'll look at it and figure out what's the best thing to do, but to me that's totally unacceptable."
The NBA fan code of conduct says, "Guests will not engage in fighting, throwing objects or attempting to enter the court, and those who engage in any of these actions will immediately be ejected from the game."
This isn't the first time Westbrook has gotten into a dispute with a fan during an NBA game. He was fined $25,000 in 2016 for directing inappropriate language toward a fan.
Westbrook, 29, had 20 points, 21 assists and nine rebounds in the loss. Paul George scored a game-high 43 points for the Thunder.