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.
It has been a while since Eric Ten Hag left the Ajax team, two years is a long time for many changes to have been made and some players also left while some where added to the team. I don’t think he can really tell how the team’s performance is actually and will have a lot of information about the team that will make him take advantage of that and be ahead of them when they meet in the Europa League. He should focus more on building a more formidable squad that can withstand any team they face in all competitions they’re going to be participating in. Ajax is not a small team, there are several others of them in the Europa league also that they might face, so they should try and channel all of those tactical decisions into winning in all games they’ll be playing in the next season. Because in the end, that’s all that matters most, they should win and be successful in all of the games they’ll play in next season and not just winning Ajax to be their goal.