If Manchester United faces Ajax, in my opinion, Erik Ten Hag holds the key to the club's weaknesses because he has coached that club before and Erik Ten Hag will definitely prepare an appropriate strategy to beat the Ajax club. However, this could also happen otherwise. Ajax has experienced Erik Ten Hag's coaching system and already understands Ten Hag's characteristics and perhaps this character can be read by the Ajax club so that this could make it easier for Ajax to beat Manchester United. In my opinion, this depends on how Erik Ten Hag handles his club, even so I still support Manchester United compared to Ajax.
What you have described is absolutely what the expectations should be but the challenge will be in if that will later turn out to be the actual reality with Manchester United and Ajax. From the time ten hag left Manchester United till this point, I believe some changes have been made and so he cannot be sure completely if he's still got intact all it takes to easily get ahead of the team, rather he may only try his best and use the little information he's got along side some relativity assumed looking at what applies currently with the team.
That should be the case because the current Mancehster United coach knows Ajax very well, so he will probably know the weak points they have. However, the problem is that Ajax is only one of many teams they will face, so they don't just focus on one team but they have to focus on many teams. In the English League alone, they have to face 17 other teams there, so they have to think about every weakness of the team they will face.
Not to mention that they will face many teams in the European League and other domestic competitions, including facing teams in the second division after the English League. A team will be very complex when talking about the opponents they will face, because it is not just against one team.