The 24-year-old is said to have been racially abused by at least one Chelsea supporter when collecting the ball for a corner during the first half of the match at Stamford Bridge.
Sterling has since posted a message on Instagram criticising the British media for ‘fuelling racism’ and Chelsea have suspended four supporters from attending matches.
Onuoha has backed Sterling for the way he reacted at the time but believes similar incidents are still occurring all the time in football.
"I think the incident with Raheem [Sterling] is terrible, but I don't think it's the only incident,” Onuoha, told the Blue Moon podcast.
“I think it's the biggest one for a while because it involves a British player. This stuff goes on all the time.
"When you look at it and think of where we are in society, it seems so wrong. On the flip side, though, I think things are still being said but more people know now it's wrong.
“Yet they still have those views and they're more than willing to share it in an open forum, whether it's in a stadium, online or face-to-face. That's the biggest problem."
Onuoha, who now plays his football in MLS with Real Salt Lake, has also suggested that he believes Sterling was unable to respond to the slurs because of the consequences he would have faced in the aftermath.
"I think one thing that a lot of people don't realise is that when stuff like this goes on for people like Raheem, he is going to lose more than those fans are going to lose if he reacts to it,” he added.
"Even though the things that are being said to him are unfair, not right, some of it is allegedly discrimination. It's horrible.
"If he were to say anything back, he'd be getting charged by the FA, the PFA would be speaking to him, Supporters Trusts and stuff like that would be getting at him.
"His reputation would be far worse, so he has to just accept it. That's one of the downsides about playing the game - you can never really defend yourself."
The former City, Sunderland and QPR defender also opened up on his own experiences of racism in the game.
Onuoha was born in Nigeria and says he and his family were victims of racist abuse when he was growing up in Manchester.
"Being raised in Manchester in the 1990s, I did experience a few things - my whole family did,” Onuoha added.
"We came from Nigeria and we were, at the time, one of the few - if not the only - black family in Miles Platting at that time.
"Time passed and I joined Man City's academy when I was 10.
"We played in a few international tournaments and some of the places we went and some of the teams we'd play, some of them couldn't speak a word of English except for being able to swear at you or say a 'n-word' to me.
"It was strange. I just thought to myself, 'What is this? Is this what I am? Is this what the world is?'
"In fairness, it got better over time and I think a lot of that was because people's voices started to be heard.”