As big a test for ETH as implementing a new playing style and getting results is how he handles this Ronaldo debacle.
I believe he went into the pre-season with good faith saying Ronaldo was important, would bring something to the team etc. but he must see now that the situation is unsalvageable, and that keeping around someone around the club with Ronaldo's temperament is poison.
How he deals with it will set him apart from our previous managers, who have either gone to war with players or done nothing and lost all authority. I'd go as far to say it could even define how his entire tenure pans out, as it could genuinely result in him losing the dressing room if he fecks it up, at a time when he's trying to win hearts and minds.
I hope whatever his view his, the club have the sense to back him.
My own view is get rid, ASAP. Any gain from keeping him around either as a first team player or squad player is outweighed by a bigger loss. In the former, the impact on our overall shape and tactics (and maybe on how hard the team work overall if they see him slacking) or in the case of the latter, disruption in the dressing room and the media storm from a sulking, stropping Ronaldo on the bench.