The hosts must win in order to keep their hopes of reaching the knockout stages for the first time alive, while Union continue to seek a first win since August in any competition.
Match preview
Braga enter this matchday five encounter knowing if they can better Napoli's result at the Bernabeu, then it will go down to a matchday six showdown in Naples.
Victory here will also guarantee a Europa League spot, eliminating Union in the process, but a defeat for the home side will end their hopes of reaching the last 16 of this competition.
A fine recent record against German opposition could give Artur Jorge's side some confidence, as they won at the Olympiastadion in the reverse on matchday two to make it four wins from five versus Bundesliga clubs.
Despite going 2-0 down in the first half, Braga roared back, and two sensational goals from Bruma and Andre Castro, with the latter coming in stoppage time, turned the game around for a 3-2 win.
The two sides also faced off in last season's Europa League group stage, with Braga winning 1-0 here.
Attacking has undoubtedly been the strong point of Braga's play, as they have found the net in all but one of their 22 matches so far this season.
They continued that run at the weekend by thrashing Portimonense 4-1 in the Taca de Portugal to reach the last 16.
Braga are far and away the top scorers in Portugal with 30 goals already - seven more than any other side, and double the tally of Porto.
That suggests an Union Berlin side who have only kept one clean sheet all season - against fourth-tier Walldorf in the first round of the DFB-Pokal - may struggle to keep the Portuguese side out here.
The German club remain in the midst of the worst winless run in the club's history, and it entered a 15th game by drawing with Augsburg at the weekend.
Their hopes of claiming third place were slightly boosted when they took a surprise point away in Napoli on matchday four, which in turn ended a 12-game losing streak.
It was not enough to save Urs Fischer's job though, as the legendary manager that took them from the lower leagues to the Champions League parted company with the club sitting bottom of the Bundesliga.
Marco Grote took temporary charge at the weekend as Kevin Volland netted a late equaliser to earn them their second draw in three games, but Nenad Bjelica was installed as the club's new manager on Sunday.
Top two is not possible despite their draw at the Maradona, and a defeat here would eliminate Union from Europe entirely.
A draw would keep hopes alive but they would be unlikely as they would then need to beat Real Madrid to have any chance, while a win in Portugal would take Union above Braga heading into matchday six.
They would need to score to stand a chance of doing that though, and that is something they have struggled to do many times this season - drawing a blank 10 times during this current winless run.
Braga Champions League form:
W
W
L
W
L
L
Braga form (all competitions):
D
D
W
L
W
W
Union Berlin Champions League form:
L
L
L
D
Union Berlin form (all competitions):
L
L
L
D
L
D
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Team News
Jorge has a fully fit Braga squad to choose from with no known injury absentees or suspensions to deal with.
There was some rotation for the cup win at the weekend, but there is no doubt that Jorge will go full strength for this one.
That means Cristian Borja and goalkeeper Matheus should come back into the side, as well as Bruma, who was left out entirely.
The biggest question mark will be in attack, as Ricardo Horta has been starting as a false nine on occasion recently, but 10-goal Simon Banza was rested at the weekend so may start here.
It will be between Andre Horta and Rodrigo Zalazar for a starting spot in central midfield alongside Joao Moutinho and Vitor Carvalho, while Rony Lopes will hope scoring on his return to the squad can boost his chances of a rare start.
Bjelica could be able to call upon trio Danilho Doekhi, Sheraldo Becker and Brendan Aaronson, who are all in contention after missing the weekend clash with Augsburg.
Leonardo Bonucci will miss out with a hamstring injury though; the club are yet to win a game since the Italian defender joined in late August.
In midfield, Union will be strengthened by the returns of Rani Khedira, from a domestic suspension, and Andras Schafer, who made his comeback following six months out with injury at the weekend.
Braga possible starting lineup:
Matheus; Victor Gomez, Jose Fonte, Niakate, Borja; Joao Moutinho, Vitor Carvalho, Zalazar; Ricardo Horta, Banza, Bruma
Union Berlin possible starting lineup:
Ronnow; Jaeckel, Knoche, Diogo Leite; Juranovic, Haberer, Laidouni, Gosens; Becker, Behrens, Fofana
We say: Braga 2-0 Union Berlin
Braga will be confident in taking advantage of an out-of-form Union side to keep their hopes of the knockout stages alive, especially considering their blistering recent attacking form.