The NBA confirmed the cushion hit a woman in the second row at Sleep Train Arena. She experienced discomfort in her eyes, according to a report by USA Today, which earlier reported the league's review of the incident.
"The pillow was light and it just flew into the air backwards. I didn't really look when it went into the crowd," Rivers said at his locker before the Clippers won their Thursday home opener 104-88 against the Dallas Mavericks. "I actually stood up for a second and I was like, 'I apologize. I didn't mean to do that.'
"I tried to tell the NBA that," he added. "This has never happened before with me. I don't have a history of throwing things. It's unfortunate, but you just learn your lesson not to take your anger out on anything. I told the NBA that I apologize and that it will never happen again."
The incident occurred during a stoppage in play with 7:47 remaining in the fourth quarter of Wednesday night's 111-104 win over the Kings.
Rivers, the son of Clippers coach Doc Rivers, invited the fan who was hit by the seat cushion and her family back to the locker room area after the game and personally apologized to them before having his photo taken with the injured party.
"It was not my intention to hurt any fan. No fan should go to the game and have to worry about something hitting them in the face," Rivers said.
"She was very nice about it. The bottom line was, it was an accident. I didn't throw anything at anybody. Throwing something and hitting something that went somewhere are two different things. I try to stay clear of things like this. I mean, this fine was more than what I got for being ejected from a game two years ago."
Doc Rivers said it was an accident while addressing the incident before Thursday's game.
"It was an accident, obviously," he said. "The unit was frustrated. He kind of just grabbed it, not to throw it in the crowd. It just happened. But it's automatic, the fine. There's nothing you can do about it.
"We use those big pads. He grabbed it just to throw it over into the back row so he could sit down. He said the thing just took off on him. There's nothing you can do about it. Next time just place it. Place it. Otherwise it costs you more."
Austin Rivers had three points in 16 minutes, shooting 0-for-3 from the field and 3-for-4 from the line in the win over the Kings.
"I'm so disappointed as a parent," Doc Rivers joked about the cushion throw.
Last season, then-Clippers forward Glen Davis was fined $15,000 for kicking a seat cushion into the stands during a game against the Celtics in Boston.
ESPN's Kevin Arnovitz and The Associated Press contributed to this report.
NBA investigating Austin Rivers incident
ESPN NBA Front Office Insider Amin Elhassan discusses the incident in which Clippers guard Austin Rivers threw a seat cushion into the crowd during a game in Sacramento.