There's no doubting the fact that Erik ten Hag was an Ajax manager before his appointment as Manchester United manager but that doesn't mean that Ajax will easily dictate ten Hag's tactics when they face Manchester United. The Ajax team under Erik ten Hag was simply better than what we have now at the club but fir Manchester United, they have a team that's full world class players so I expect Manchester United to beat Ajax should they meet each other in the UEFA Europa League next season.
A very good point you have made, the fact that Eric Ten Hag was once the head coach of Ajax doesn't give either team a head on advantage utmostly, Ajax can not completely ascertain what the tactics Erick Ten Hag will be using neither will Ten Hag be able to completely tell how better he's going to be against Ajax, as at the time he headed the team as a coach there was a different tactics which I don't believe has been held into till this time, and for Manchester United they have a good team that would have been considered much better than that of Ajax but if they put their strategy together to be perfectly all inclusive then they can be sure of an edge over Ajax.
Basically, if we only compare the quality and level between Manchester United and Ajax, then surely we will choose Manchester United is the best compared to Ajax. Thus, when we add another factor about Manchester United advantage about Erik ten Hag having been the head coach of Ajax and Manchester United having better players. Then of course, the result is that Manchester United will still be the favorite team to win if for example they meet in the Europa League next season.
So, the factor of Erik ten Hag having been the head coach of Ajax in the past is certainly a factor also to be able to make Manchester United win in the match. But yes, in the Europa League any result can happen, because so far the Europa League has also become evidence when the non-seeded team managed to win when playing in the decisive match.