Looking at last season...
Points per game where he played 15 minutes or less - 2.1 (!!) (10 games, 8 of which he played 0 minutes, 21 points)
Points per game when he played more than fifteen minutes - 1.0 (28 games, 27 points)
The two games I included in the top stat are Liverpool and Brentford. So I think it's fair. However if you go purely games he didn't play vs games he did play, its 1.9 (at 0.75 goals per game conceded) vs 1.1 (1.4 goals per game conceded).
I know sometimes stats aren't taken very seriously, so I will also confidently state whenever I watched him play he looked bang average at best, and more frequently missing than doing what we needed of him.
At no point next season, when the inevitable bad result happens, will any Everton fan think 'if only Onana had still been here'.
We will look back fondly on our Top 4 levels of form last season when he was nowhere to be seen.
Good luck at Villa Amadou!