Onana, usually the midfield destroyer for the Belgians, was shifted to centre back for the start of the friendly in Rijeka, which Belgium won 2-0 with a morale-boosting performance.
It was a surprise switch from their traditional 4-3-3 formation, but one that had been planned to see whether it might be an option at the tournament in Canada, Mexico and the United States.
"I felt good in that position," Onana told a press conference on Wednesday. "The partnership with Arthur (Theate) and Nathan (Ngoy) went quite well. They carried out their roles very effectively too. For a first full dress rehearsal, this was certainly not bad at all.
"I've played there a few times before. The team comes first. I'm a midfielder, that's my favourite position, but if they need me at the back, I'll do it. Even if the coach needs me up front as a striker, I'll play there," Onana told reporters.
"I feel most comfortable in midfield, and I believe that one day I can be one of the very best players in the world in that position. The World Cup is the priority right now, though, and if I have to play in defence, I'll do it without complaining. I felt good there yesterday as well."
For coach Rudi Garcia, the experiment went well.
"This gives us more variety in our play, especially because we have many players who are versatile enough to operate in this system," he said.
"Of course, there are still things that can be improved. But at least we now know that we can use this system to start a match or switch to it during a game and that we can also achieve results with it.
"Because he is naturally more of a midfielder, it gives us the option to move him around. We saw (against Croatia) that he has all the qualities needed to do this job successfully."