Appointing a quality manager in his prime was the catalyst.
A catalyst implies that something or someone precipitates an outcome that was already expected in the first place.
The club's situation had already reached rock bottom. Ten Haag came to change the trajectory, but in his first two matches, it seems like things could still get worse.
Then he dropped Maguire and Ronaldo, who were nowhere to be found on the bench, and we beat Arsenal. Things got better after that. But...
...we still lack cohesion. City beats us to the ground. Our journey in Europe was not really convincing when Ronaldo played.
The final nail in Ronaldo's coffin was when he left the bench during Tottenham's game and refused to come. I think Ten Haag at that time has already decided that Ronaldo will be involved no more than as a squad player, and he wants to test Ronaldo's willingness to accept his new role by bringing him on in the match's final minutes.
The rest is history. Garnacho's goal, followed by Piers Morgan's interview, and finally, his exit.
So, yes, Ronaldo's leaving was the catalyst. Ten Haag has set the course of our club, but it was accelerated by the situation with Ronaldo. It allowed our best players, Rashford and Bruno, to fully express themselves.
It's not the only catalyst, mind you. Dropping Maguire is also quite important. It allows other players who are more competent than Maguire, such as Varane, Licha, Case, and Bruno, to resume the leadership role on the pitch, although maybe it is a technical decision more than anything.