That's the thing, though — to play Captain Hindsight (while making certain assumptions that might not be grounded in reality for all we know),
maybe he does not know how to successfully duplicate Ajax's system? Without discrediting what he accomplished there (one might argue that he was their greatest, or at least one of the greatest, Head Coaches since Louis van Gaal in the 1990s, and is a modern icon of the club — no one can or should deny him those decorations), maybe successfully replicating Ajax's system, seemigly all on his own, is simply a step too far for him?
Clubs like Ajax and Barcelona are a bit unique in the sense that have long-standing footballing principles (and dogmas), and a good idea of how they want to operate and play (which trickles down on an institutional level, and folk (particularly outsiders who are brought in) are compelled to fall in line). They might stray from those principles from time to him, but invariably retrace the hallowed guidelines of Cruyff and Michels. Overmars constructed a squad for Ajax style of football, young players who were trained for Ajax style of football, at Jong Ajax or elsewhere, stepped up during ten Hag's tenure (de Ligt, de Jong et cetera), Bosz had previously set the stage for Ajax style of football and even reached a Europa League final...
Ajax return to the Cruyff ideals as Peter Bosz leads new generation
In celebrating what ten Hag did with Ajax (especially during the dreamlike Champions League run), and drooling at the prospect of that being duplicated (have to admit, I was guilty of this...like countless other United supporters!
) maybe we failed to consider and surgically examine his actual expertise at replicating those principles and that brand of football when he was taken out of that environment (i.e., without comparable institutional oversight or directive or support to play Ajax style of football, without Overmars to procure the ingredients for him, without the backdrop of Bosz, and with stars like Cristiano, Rashford, Bruno — who are not optimized fits for that style of football).
Perhaps it's like signing a player who excelled in a specific “system” (with the presumption that they will produce similar results elsewhere), and finding out that they cannot actually produce similar results when taken out of that specific system, and might even underperform to near-catastrophic levels, when placed in a different ecosytem (we have witnessed this with a handful of former Brighton players and staff in recent years, when they are taken out of an environment that was meticulously developed and curated by Tony Bloom (aided by analysts and statisticians) himself).
What if, Erik has tried to wed: 1. His personal preferences (every manager has 'em, to varying degrees) with 2. Ajax style of football, in a non-Ajax environment and 3. His, possibly erroneous, interpretation of United style of football (for example, wanting to be the best transition team in the world without the appropriate level of structural solidity in midfield and defense, or industry/aggresiveness/decisiveness in general) — and created something that's neither here nor there, a jumbled mess that even he can't get to the bottom of (let alone sort out)?