The visitors have an opportunity to move top of the table by winning, as the two sides above them in the table - Bayern Munich and Borussia Monchengladbach - do not play until they meet each other later on Saturday evening.
Match preview
Schalke, meanwhile, remain winless upon their return to the Bundesliga, but they made it back-to-back draws with a 0-0 stalemate away to Wolfsburg last weekend.
Head coach Frank Kramer would have left Lower Saxony frustrated that his side could not claim all three points after Simon Terrode's woes in front of goal continued, with the all-time record Bundesliga 2 goalscorer twice missing from the penalty spot after being afforded the opportunity of a retake due to Koen Casteels being adjudged to have stepped off his line. However, the Belgian goalkeeper failed to fall for Terrode's double-bluff, as the striker twice went in the same direction to no avail.
In truth, Wolfsburg had the better of the second-half proceedings, with Joshua Guilavogui's effort ruled out for offside, before Ridle Baku missed a glorious opportunity from close range to snatch all three points.
Avoiding defeat twice in a row is a step in the right direction for Schalke at this stage in their development, but Kramer will be more than aware that his side will need to find wins at some point in the season if they are to move themselves away from relegation danger.
Bagging all three points against Union Berlin on Saturday may prove to be tricky, though, with Urs Fischer's side one of the in-form teams in the Bundesliga.
Seven points from a possible nine has continued the positive momentum generated over the past few seasons in East Berlin, with Die Eismen only falling short of Champions League qualification by a single point last campaign.
RB Leipzig were the side who pipped them to fourth spot, but Union Berlin managed to gain a small slice of revenge last weekend, with Jordan Siebatcheu and Sheraldo Becker continuing their scintillating form with a counter-attacking goal apiece before the half-time interval.
The visitors rallied after the break, with Willi Orban's close-range header giving them hope seven minutes from time, but Fischer's side held on for a crucial victory. Should they be able to find another win in Gelsenkirchen, they may well find themselves top of the Bundesliga come the full-time whistle.
Schalke 04 Bundesliga form:
Schalke 04 form (all competitions):
Union Berlin Bundesliga form:
Union Berlin form (all competitions):
Team News
Schalke remain without Amine Harit as Kramer continues to rule him out of selection for disciplinary reasons, while forward Sebastian Polter is out with a thigh injury.
Salif Sane is unlikely to feature due to a lack of match fitness having been plagued by injuries in recent seasons, but Tobias Mohr could return after missing out against Wolfsburg.
The former Heidenheim winger may slot straight back into the starting XI in order to provide a more attacking outlet, with central midfielder Danny Latza likely to drop to the bench.
Union Berlin, meanwhile, do not appear to have any injury concerns, although Timo Baumgartl remains unavailable as he continues his recovery from testicular cancer.
As such, it seems extremely unlikely that Fischer would change a winning formula from last weekend's impressive victory over Leipzig, with Becker and Siebatcheu continuing in tandem in attack.
Schalke 04 possible starting lineup:
Schwolow; Brunner, Thiaw, Yoshida, Ouwejan; Krauß, Kral, Zalazar; Mohr, Terrode, Bulter
Union Berlin possible starting lineup:
Ronnow; Jaeckel, Knoche, Leite; Trimmel, Schafer, Khedira, Jaberer, Ryerson; Becker, Siebatcheu
We say: Schalke 04 1-2 Union Berlin
With the visitors preferring to play in transition, we can envisage them gaining another victory on Saturday to potentially move top of the table.
Schalke have remained competitive in the early weeks of the campaign but could find themselves caught between two stools in front of a demanding home crowd.