Opponents who used to press us high due to DDG's inability with the ball will think twice about that as they could get exposed with someone like Onana who is composed in possession.
We suffered too much possession with DDG launching high rockets without any purpose. Onana is capable of finding players centrally, if space is open due to not coordinated press by an opponent, or making a chipped pass to wide areas where we will have a free man if the opponent stays narrow to prevent being outnumbered centrally.
Also, he is capable of finding a target with long passes if necessary, as opposed to DDG's pointless launching.
Stylistically, we will have more of the ball and more sustainable possession than we had last season. We will probably still be short of a few players (CM & CF) but gradually our chance creation will depend less on the fast turnovers as we will start to structurally build our attacks from the first phase.
We will finally have a use of an extra man in the build-up and it really can transform our play.