I don't think Ajax was favorite in this match. Ajax may have played well in the group-stage matches. But now Ajax is not in good form. And this was proved once again in Ajax's performance in this match. Ajax's overall performance is very poor. Ajax players did not try to attack in the first half against Union Berlin. I have never seen an Ajax team play so badly defensively before. They made several attacks in the second half. But in the full 90 minutes, they were not able to make a single shot on target. Ajax failed to win this match due to the poor performance of the forward players.
Ajax was still the big favorite in the match against Union Berlin even though Ajax is not in a good shape at the moment. We should not forget where Union Berlin is coming from. They were a part of the second Bundesliga in Germany for many years before they managed to go up to the first Bundesliga only a few years ago. Usually such clubs are fighting to not be relegated down again during almost every season. Ajax on the other side was in the semi final of the Champions League not to long ago and they have almost reached the final back then.
Somehow though Union Berlin manages to perform in almost every game so far in this season.