Following Arsenal, yes I was. I believe most Arsenal fans would agree as well
Yup, that was when the team first really took off. There were a lot of positive signs beforehand, but huge inconsistency. I would periodize the early Arteta years like this:
December 2019 - December 2020
Arteta takes over, plays around with different tactics before settling on a very conservative setup that wins the FA Cup but then runs into massive problems the next fall as the team has zero cutting edge and is incredibly ponderous moving the ball up the pitch. We can't get many new players due to Covid and finances, the one big purchase in Thomas Partey is immediately hurt and hardly plays, the veteran addition of Willian who was supposed to supply us with more creativity is a total disaster.
January 2021-November 2021
Arteta decides to throw caution to the wind and open things up. This leads to some much more positive play and we first start to see the kinds of patterns in possession that will eventually be a hallmark of the team. But we really can't counter-press or control transitions at all because the CB group is still awful and can't play in space (David Luiz, Holding, Gabriel, Chambers, Pablo Mari). We then buy Ben White and Tomiyasu over the summer and for the first time can field a decent backline that can play higher up the pitch and counter-press more aggressively, but inconsistency still plagues us as the squad is really learning a new way of playing.
December 2021 - May 2021
The big leap forward you identified. Dropping Auba and bringing in Martinelli is a big part but its also just continuing to learn to play the aggressive high line possession tactics that we started implementing a few months earlier. There are still some very big issues. Our CF is Lacazette and can't really threaten anybody, our only half decent LB doesn't really fit the system and can't stay healthy, and the team runs about 11 deep and as soon as anybody is injured the system starts to break down, which happens at the end of the season.
There is still a long way to go after that obviously.
In terms of Ten Hag, I think a generous interpretation of his United tenure is that he is in his own version of the January 2021-November 2021 period, in which he's trying to play more positively but he doesn't have the defenders or defensive midfielder to really control matches while playing high up the pitch, so you have lots of inconsistent results and frustrating games.
The critical interpretation would be that (a) Arsenal in that period of first embracing a more open style of play were still better by almost any metric (goal difference, points, XG) than United were this past year when doing the same thing and (b) Arteta did that while spending a lot less money by the end of that period than Ten Hag has spent by this point. Arteta's teams really got better once he was able to truly spend substantial amounts of money and buy players that fit his system, which didn't really happen in earnest until the summer of 2021 (bringing in White, Tomiyasu, Odegaard, Ramsdale). Ten Hag has spent more money and had worse results when trying to move to a more open style.
