Not sure how closely you followed the details of that saga but it's generally accepted that Ten Hag was told Greenwood was coming back.That is a very generous to EtH interpretation of things. You sound as if Greenwood were some ready-made world class player who was ready to drag the team on his own, not a young still developing player after 18 months of no football. Even if he were a world class player - pinning the “entire squad planning” on him would be unwise to put it mildly.
I do not believe for a moment that it was communicated to EtH that “Greenwood is coming back 100%”. It was probably “likely” or “we will do our best”. Even Murtough is not that stupid.
Even if it (stupidly) was communicated like that - EtH should not have been that naive to rely on him coming back to such a significant extent. Again, given his age and long absence a squad rotation player is an absolute maximum you could reasonably expect.
If he were suspended weeks/days before the end of the transfer window- different story and you can maybe use it as excuse is a manager. But this excuse could only be used by Ralf, not by EtH.
No serious organisation would accept “I just relied on X giving me assurances that something good is likely going to happen, therefore no check, no contingency plan” from an employee with a position of such authority on a performance review regarding a critical issue. He is paid 9m a year ffs.
Whether it was wise to rely on him after 18 months out is a valid question, but actually irrelevant to the point being made about failures at board level having a major impact on Ten Hag's squad planning. In particular upfront where we have been lacking numbers for the whole of Ten Hag's tenure.
The greatest failure with Greenwood was simply the constant delays from Arnold in announcing anything, this meant that it was just days before the transfer window closed that the club U-turned and finally announced he was leaving.
This left no time for any replacement and has had a major knock on effect. In fact it might be that FFP restrictions essentially meant that Ten Hag was given the option of Greenwood or nothing, before at the last minute being told nothing was the only option!