Geneva: Even by Champions League standards, the stakes are dramatically high for Fenerbahce and Arsenal in the first leg of the playoff round on Wednesday.
Arsenal seem to be in turmoil after a Premier League opening day loss fuelled fans' fury at the cash-rich club's failure to sign big-name offseason targets.
"I believe our season will depend on how we respond to this defeat," Arsenal manager Arsene Wenger said on Saturday after a 3-1 loss to Aston Villa provoked hostile boos at Emirates Stadium.
Fenerbahce host Arsenal in Istanbul only hours after their lawyers begin a two-day court hearing in Switzerland to challenge their 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 in UEFA prize money.
Like Arsenal, AC Milan regard a place in the Champions League group stage as key to their image and finances.
The seven-time European champions begin their season Tuesday away to PSV Eindhoven, the 1988 European Cup winners who have already reeled off three wins to sit atop the Dutch league.
Zenit St. Petersburg offer a tricky debut for Champions League newcomers Pacos de Ferreira, who host the first leg in Portugal on Tuesday.
Also on Tuesday, Celtic have made the six hour-plus flight to face Kazakhstan champions Shakhter Karagandy, Lyon host Real Sociedad and Viktoria Plzen host Maribor.
Wednesday's five-match lineup includes: Dinamo Zagreb vs. Austria Vienna, Steaua Bucharest vs. Legia Warsaw and Ludogorets Razgrad vs. Basel.
Schalke host PAOK Thessaloniki, who were reinstated by UEFA last week. PAOK previously lost to Metalist Kharkiv, who were later expelled when a 2008 match-fixing case was resolved this month.
Despite two legal defeats at CAS already, Metalist seek 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 on Monday.
Three of the 10 playoff pairings have been affected by UEFA match-fixing investigations since current champions Bayern Munich beat German rivals Borussia Dortmund at Wembley Stadium in May.
Fenerbahce's case has the greatest potential for drama. The club, UEFA and CAS could not process their 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 know they 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 on Saturday to join defenders Thomas Vermaelen and Nacho Monreal, plus midfielders Mikel Arteta and Abou Diaby on the sidelines.
Milan, which begin their Serie A campaign at Verona on Saturday, have 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 their 4-0 loss at Wolfsburg in the Bundesliga 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 champions Steaua, the 1986 European Cup winners, host Legia while club president Gigi Becali serves prison time for financial wrong-doing.
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 on August 29 in Monaco.