Villa are in Belgium for the first-leg against Club Brugge tonight.
Onuoha told BBC Sport: "I think Unai Emery himself has felt very comfortable being in Europe, because we have seen him be successful in Europe with different sides before and I think that’s made a big difference in how they have equipped themselves in the Champions League this year.
"I think the bigger issues have come with the way they bounce back after a midweek Champions League game.
"I think they’ve tended to lose those games afterwards.
"Given the fact that we are heading towards the end of the season, they’re languishing 10th in the Premier League and even though it is a few points away from being in the top four, top five that will be a concern for them but the players that are there, they’ve got the belief that they can go and be successful."