It is the strength of these two clubs and game patterns that will determine who will win the match because Ten Hag knows the capability of each player in Ajax so that is an advantage to him but the other coach doesn't know more about Manchester united players.
I will not be able to say who will win the match until the season starts and let's see the strength of the new Manchester united team next season that is when I will be able to predict who will win if they clash.
Having left Ajax for several years I don't think we can say Ten Hag holds Ajax's weakness just because he was once Ajax's coach. Some players have changed and each coach will have different game tactics. But Ten Hag can still understand the team's culture and playing patterns because he was once a coach. And I think the preparation of both teams will be the difference when they clash later.
For the record, currently Ajax and Manchester United have almost the same record last season. Ajax's performance was not that good last season. It could even be said to be one of the worst performances for Ajax in the Eredivise. Manchester United is also in the same situation. They wouldn't have even qualified for the UEL had they not managed to win the FA cup.