If you look at the previous five matches, Union Berlin's chances of winning this match are much greater than Leipzig because so far they have only experienced 1 defeat in their previous five matches. The defeat they received was also quite reasonable because they faced Bayern Munich as the top of the standings. Of course, this match will be very important for Union Berlin and even though it is difficult to win, at least a draw is enough for them not to lose full points.
Every match in the next weeks and til the season ends is now important for Union Berlin.
For sure Leipzig hasnt shown there best face the last 2 weeks but it will be hard to get anything out of Leipzig.
For Leipzig its important too , to win that game to catch up to the other teams and maybe have a chance to get close to an CL place.
Although in some recent matches that Union Berlin got better results compared to Leipzig but still, their position in the standings is very far apart. because yes, Leipzig is in the upper zone and Union Berlin is in the lower zone and moreover, Leipzig also plays at home. Of course, with these factors, Leipzig still has a good chance of winning. But indeed, I also recognize that although Union Berlin is in the lower zone in the current standings but indeed, that Union Berlin is also in a good trend to stay away from the relegation zone. But to be honest, beating Leipzig at home remains a difficult mission for Union Berlin in this match.