There is more to being a manager than early good performances through. The primary and immediate thing he changed was allowing players to attack in greater numbers, with less caution. This is riskier defensively except if you tell your players to never over commit as Mourinho pretty much always did, eventually your defence collapses because the other side applies relentless pressure due to lack of their own need to defend. When you attack at an insane pace you keep the opposition off balance and for the most part attack being the best form of defence holds true. Defend too much and you're begging for trouble.
But longer term maybe he's crap at training, can't handle players egos, players get upset at not being played and he sucks at dealing with that, terrible judge of players when it comes to transfer market, etc. All of that is relatively unknown. The main thing he's done so far is come in and release the offensive players to play their actual game rather than put a leash on them and place relentless pressure/negativity on them by insisting they defend all game.
Early signs are he's doing great, longer term he could be great or could be terrible. Short term uplifts by fixing the major issues holding a team back isn't rare, long term a new manager who got such a uplift actually making a team better is rarer.
