Frankfurt, Germany — Borussia Dortmund can qualify for the knockout stage of the Champions League if they beat Galatasaray in Group D on Tuesday.
Then it can focus on staving off relegation in the Bundesliga.
Arsenal, too, will be assured of advancing if it beats Anderlecht at home and Dortmund defeats — or draws — the Turkish side.
Dortmund has won all three matches so far without conceding a goal and has nine points. Arsenal lost to Dortmund but won the next two and is second with six. Anderlecht and Galatasaray have one point each.
Here are some things to know about Tuesday's matches.
Despite its perfect record in the Champions League, Dortmund is in big trouble in the Bundesliga.
Dortmund has dropped to next-to-last after losing 2-1 at Bayern Munich on Saturday, its fifth consecutive loss. Juergen Klopp's team has lost seven of its first 10 matches in the Bundesliga season.
The last time Dortmund lost seven of its first 10 was 30 years ago.
Dortmund is even on points with last-place Bremen, which has scored more goals. Dortmund, Champions League finalist in 2012 and winner in 1997, is 17 points behind Bundesliga leader Bayern and 13 points away from qualifying for next season's Champions League.
One thing to remember, though: When Dortmund began the Champions League with three straight victories, it won the title, in 1997.
DILEMA IN DEFENSE
Dortmund central defender Mats Hummels injured his foot in Munich and will be out for several weeks.
Hummels left the game at halftime and then watched his replacement, Neven Subotic, make two crucial mistakes than led to Bayern's come-from-behind victory.
Hummels and Subotic formed the backbone of Dortmund's championship sides in 2011 and 2012. But Subotic has been slow in regaining form after sitting out more than half a year with a knee injury.
Hummels came back from this year's World Cup in Brazil with the title but also with an injury and missed the start of the season. Just as he began returning to form, he's now out again.
Klopp could decide to play the experienced Subotic against Galatasaray or give another chance to Matthias Ginter. Ginter was in the World Cup squad and was signed away from Frieburg, but has not had a start since giving away an own-goal six weeks ago.
GOOD IN GERMANY
Galatasaray's 4-0 home defeat to Dortmund gives it little ground for optimism going to Germany but for one fact: the Istanbul side has always performed well there.
Galatasaray has won five in Germany, with five defeats and three draws. With Germany's large Turkish immigrant population, Galatasaray has always enjoyed a lot of support there.
Galatasaray has won in four of its last six trips to Bundesliga opposition, losing only once.
Five Galatasaray players were born in Germany: Hamit Altintop, Hakan Balta, Tarik Camdal, Furkan Ozcal and Yasin Oztekin.
Dortmund has three players with Turkish roots — Nuri Sahin (injured), Ilkay Gundogan and Burak Camoglu.
ARSENAL EYES KNOCKOUT STAGES
Arsenal can secure its place in the second round if it beats Anderlecht on Tuesday and Galatasaray does not beat Borussia Dortmund.
Arsenal forward Alexis Sanchez has scored four times in his last two games, spearheading a strike force boosted further by Theo Walcott returning from a torn knee ligament that has kept him out since January.
Walcott made a substitute appearance in Arsenal's 3-0 victory over Burnley on Saturday, and will want to step up his comeback on Tuesday.
Arsenal manager Arsene Wenger will hope for a smoother game than the last one against Anderlecht, where two goals in the final two minutes of the match in Belgium secured a dramatic 2-1 victory.
If Arsenal already dashed Anderlecht's confidence with a belated 2-1 come-from-behind win in Brussels last month, injuries and lackluster form in the Belgian league have only added to its low morale since.
The Belgian champions travel to London without two key players. Belgium international Steven Defour, Anderlecht's midfield mainstay, injured his calf during a mediocre 1-1 home draw against Lokeren.
Even worse, creative forward Matias Suarez will be out for the rest of the year with a knee injury. It leaves Anderlecht without its star players in two lines, and its attack has been a problem for most of the season.
In the league, Anderlecht lead by one point over AA Ghent and two over FC Brugge, with only 27 points out of a possible 42.
The Champions League has given it little pleasure, with only a draw against Galatasaray giving some relief. Anderlecht has finished bottom of the group in all seven group-stage campaigns since it reached the last 16 in 2000-01.