Leipzig has lost 3 matches in a row and they have not been able to win a single match in the last 4 matches. So they are definitely struggling. But they will look at this match as a way of getting back into form and this match is going to be essential for them to actually get close to Dortmund. If they can win this match they will be only one point behind Dortmund.
So this is a great chance for them to get 3 points and get back in form. Union Berlin has been really inconsistent and I don't think it is going to be a good idea to trust them right now. But I believe this match can go either way because both teams have been pretty bad recently. I also won't trust Leipzig at this moment, to be honest.
Leipzig lost a lot of points to stay in the race for the top three and the inconsistency of the game they showed meant that a number of opportunities were lost in vain. Even though they managed to beat Union Berlin with a score of 2-0, Leipzig's position was still below Dortmund because they had a difference of one point. If Leipzig is not able to play much better then it is quite worrying that they will finish this season below the top five in the standings so they will not qualify for the Champions League.
Union Berlin is also experiencing setbacks and they are having difficulty getting wins, currently Union Berlin's position is quite at risk of relegation and if their performance does not change it will be very dangerous. Two points above Klon in the bottom third of the standings and this shows that their position is not secure until the end of the season, especially if they show no changes in the future.
Yes, Leipzig has lost a lot of points in the last few matches. Leipzig has experienced 3 consecutive defeats, 1 draw and 1 win, where Leipzig has lost 11 points. Therefore Leipzig is quite far behind the top 3 clubs. Even though Leipzig still has the opportunity to occupy the top 3 rankings its performance Leipzig's is are still quite doubtful and unstable, but at least Leipzig can break the series of bad results in the last few matches by winning the match against Union Berlin in week 20.
Union Berlin not only experienced a slight setback but experienced a very drastic decline, in fact I didn't expected Union Berlin to experience a very bad season this season because last season Union Berlin was ranked in the top 4, while this season Union Berlin had to fight to avoid the relegation zone because his current position is very insecure with clubs that are in the relegation zone because he is currently rankings 15th out of 18 clubs.