The Bundesliga side are currently bottom of the section on zero points, having lost their opening two games to Real Madrid and Braga, while Napoli are second, picking up three points from an available six.
Match preview
Union Berlin finished fourth in the Bundesliga last season to secure a position in the Champions League for the first time in their history, but they have suffered late heartbreak in their opening two games in the competition.
Indeed, Jude Bellingham scored a last-gasp winner for Real Madrid at Bernabeu on matchday one, before the German side threw away a two-goal lead against Braga last time out to lose 3-2, with the Portuguese team netting their third in the 94th minute of the contest.
There is no getting away from the fact that it has been a hugely difficult start to the season for the Iron Ones, who have lost eight of their 11 matches in all competitions.
Union Berlin actually started the 2023-24 campaign with three straight wins, but they have lost all eight since, including defeats to Borussia Dortmund and Stuttgart in the Bundesliga since their last European fixture, with the team currently sitting down in 15th position in the Bundesliga table.
Urs Fischer remains in charge of the club, and the 57-year-old desperately needs a big result on Tuesday, but this is the team's worst run of form since the 2004-05 campaign.
Napoli have not exactly been spectacular themselves in the competition this season, needing a late own goal to beat Braga on matchday one, before suffering a 3-2 home defeat to Real Madrid last time out, but they are second in Group C, level on points with third-placed Braga.
The Blues lost back-to-back matches to Real Madrid and Fiorentina before the international break, which led to speculation surrounding Rudi Garcia's future, but the head coach has kept his job for now.
The Italian champions managed to record a 3-1 win over Hellas Verona in Serie A on Saturday, with the result leaving them in fifth position in the table, five points behind the leaders Inter Milan.
Napoli have won eight, drawn five and lost nine of their previous 22 European contests with German opposition, while their away record is three wins, three draws and five defeats.
The 2022-23 season proved to be the club's most successful European campaign, as they reached the quarter-finals of the competition before losing to domestic rivals AC Milan, while Gli Azzurri have been victorious in seven of their last nine matches in the group stage of the Champions League.
Union Berlin Champions League form:
L
L
Union Berlin form (all competitions):
L
L
L
L
L
L
Napoli Champions League form:
W
L
Napoli form (all competitions):
D
W
W
L
L
W
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Team News
Union Berlin will be without the services of Alexander Schwolow, Andras Schafer, Laurenz Dehl, Josip Juranovic and Jerome Roussillon for Tuesday's European clash due to injuries.
However, Robin Knoche, Rani Khedira and Alex Kral all featured on Saturday following fitness issues, and the trio are expected to be available for selection in this match.
Kevin Behrens and Robin Gosens have scored four times in each this season and will both be in the home side's XI on Tuesday, while Sheraldo Becker, who has registered three goals this term, should also start.
As for Napoli, their main team news surrounds Victor Osimhen, with the team's star player unavailable for the next few weeks due to a thigh problem which he suffered on international duty with Nigeria.
Andre-Frank Zambo Anguissa and Juan Jesus will also be absent for the Italian side due to injury issues.
Khvicha Kvaratskhelia scored twice against Verona on Saturday and is again expected to be joined in the final third of the field by Matteo Politano and Giacomo Raspadori.
Union Berlin possible starting lineup:
Ronnow; Doekhi, Bonucci, Leite; Knoche, Tousart, Kral, Laidouni, Gosens; Becker, Behrens
Napoli possible starting lineup:
Meret; Di Lorenzo, Rrahmani, Natan, Olivera; Zielinski, Lobotka, Cajuste; Politano, Raspadori, Kvaratskhelia
We say: Union Berlin 1-2 Napoli
Union Berlin desperately need a positive result, but we are finding it difficult to back them to avoid another defeat here. Napoli are without their star player, but the Italian outfit are still full of quality, and we are expecting them to navigate their way to an important win.