Arsenal seems to be in turmoil after a Premier League opening day loss fueled fans' fury at the cash-rich club's failure to sign big-name off-season targets.
"I believe our season will depend on how we respond to this defeat," Arsenal manager Arsene Wenger said Saturday after a 3-1 loss to Aston Villa on Saturday provoked hostile boos at Emirates Stadium.
Fenerbahce hosts Arsenal in Istanbul only hours after its lawyers begin a two-day court hearing in Switzerland to challenge its expulsion from the Champions League by UEFA in a long-running, domestic match-fixing case. UEFA agreed to let Fenerbahce play while the appeal proceeds.
The Court of Arbitration for Sport's verdict — promised next week after the return match in London — could yet decide which team advances to the lucrative group stage, and collects 15-25 million-plus euros ($20-$33 million) in UEFA prize money.
Like Arsenal, AC Milan regards a place in the Champions League group stage as key to its image and finances.
The seven-time European champion begins its season Tuesday away to PSV Eindhoven, the 1988 European Cup winner which has already reeled off three wins to sit atop the Dutch league.
Zenit St. Petersburg offers a tricky debut for Champions League newcomer Pacos de Ferreira, which hosts the first leg in Portugal on Tuesday.
Also Tuesday, Celtic has made the six hour-plus flight to face Kazakhstan champion Shakhter Karagandy, Lyon hosts Real Sociedad and Viktoria Plzen hosts Maribor.
Wednesday's five-match lineup includes: Dinamo Zagreb vs. Austria Vienna, Steaua Bucharest vs. Legia Warsaw and Ludogorets Razgrad vs. Basel.
Schalke hosts PAOK Thessaloniki, which was reinstated by UEFA last week. PAOK previously lost to Metalist Kharkiv, which was later expelled when a 2008 match-fixing case was resolved this month.
Despite two legal defeats at CAS already, Metalist seeks another ruling from Lausanne by midday Tuesday (1000 GMT) to get back into the competition "on the basis of alleged new evidence," the court said Monday.
Three of the 10 playoff pairings have been affected by UEFA match-fixing investigations since current champion Bayern Munich beat German rival Borussia Dortmund at Wembley Stadium in May.
Fenerbahce's case has the greatest potential for drama. The club, UEFA and CAS could not process its challenge to a two-season ban from European competitions before the matches against Arsenal.
During an off-season legal saga, Fenerbahce forward Dirk Kuyt wrote to UEFA on behalf of the players. Kuyt said they had "a lot of questions" about the punishment dating back to matches allegedly fixed in the 2010-11 Turkish season.
"This is not even proven that we are guilty," wrote Kuyt, whose club president was among several officials convicted in a criminal trial. They deny wrongdoing and have appealed.
Injury-hit Arsenal knows it could lose over two legs and still earn a 16th straight season in the group stage — though Salzburg, Fenerbahce's beaten opponent in the previous qualifying round, could also seek reinstatement.
England winger Alex Oxlade-Chamberlain is the latest Gunners casualty, seriously injuring his knee Saturday to join defenders Thomas Vermaelen and Nacho Monreal, plus midfielders Mikel Arteta and Abou Diaby on the sidelines.
Milan, which begins in Serie A at Verona on Saturday, has played only friendlies ahead of facing match-fit PSV, now coached by former Netherlands midfielder Phillip Cocu.
Schalke lost Dutch forward Klaas-Jan Huntelaar to a knee ligament injury in its 4-0 loss at Wolfsburg on Saturday.
The revised fixture with PAOK brings the Greek club's coach Huub Stevens back to Schalke, where he won the UEFA Cup in 1997 and at which a second spell in charge ended last December.
Romanian champion Steaua, the 1986 European Cup winner, hosts Legia while club president Gigi Becali serves prison time for financial wrongdoing.
In June, UEFA weighed allegations that Becali tried to pay bribes to influence a 2008 league match, and deferred Steaua's one-year suspension for a five-year probation period.
The 10 playoff winners join 22 elite teams that qualified directly for the lucrative group stage. The draw is made Aug. 29 in Monaco.