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.