For context, Bournemouth outplayed City last night with Adam Smith at right back and James Hill at centre back. Neither of them are household names or cost a lot of money. Brighton have had the likes of Dunk, Webster and March in their team in recent years.
"Premier League" quality means being able to fulfil your role in the team. You don't need to be a superstar, just a competent footballer. When O'Nien makes a mistake, people jump on in a way that they don't with other players. He didn't play well against Villa but he has performed perfectly adequately in his other appearances. He is a squad player and happy to accept that role while helping to set mentality in training and games.