Barcelona’s Luis Suarez celebrates after scoring against Bayer Leverkusen at Camp Nou in Barcelona on Sept. 29.Alex Caparros/Getty Images
Jose Mourinho's season took another turn for the worse on a night of surprises in the Champions League.
With Chelsea floundering in 14th in the English Premier League, a win at Porto on Tuesday would have been a much-needed boost, but instead Mourinho watched a 2-1 defeat at the hands of the club he led to the Champions League title in 2004.
Mourinho said his team paid the price for "two ridiculous mistakes" in defence as Yacine Brahimi and Maicon's goals powered the Portuguese team to victory.
Arsenal fared no better than its London rival, losing 3-2 at home to Olympiacos. That leaves Arsenal bottom of Group F with rapidly shrinking hopes of qualifying for the knockout stages. Another surprise was in store in Belarus, where BATE Borisov upset AS Roma 3-2.
However, Barcelona pulled off a dramatic escape to avoid defeat against Bayer Leverkusen, scoring in the 80th and 82nd minutes to turn defeat into a 2-1 home victory.
"We are the European champions and we knew that in the second half we had to show it," said Luis Suarez, who scored the winner.
Here is a look at Tuesday's Champions League matches.
Group E
Barcelona's dramatic comeback was achieved without Lionel Messi, who is set to miss two months with a torn knee ligament, and the team's injury worries grew when Andres Iniesta was forced off in the second half with an apparent thigh injury.
Sergi Roberto poked in the rebound of a strike by fellow substitute Munir El Haddadi to equalize in the 80th, and two minutes later El Haddadi passed for Luis Suarez to drive in the winner, sending Barcelona to top of the group.
Leverkusen outplayed the defending champions for an hour and looked set to take revenge for its 7-1 loss the previous time the German side visited Camp Nou in 2012, when Messi scored a record five goals.
Defender Kyriakos Papadopoulos scored the opener in the 22nd, but Leverkusen faded late and didn't have the legs to keep up with Barca's reinforcements.
Also in Group E on Tuesday, BATE Borisov beat Roma 3-2.
BATE was widely expected to be a serial loser in Group E, but two goals from left-back Filip Mladenovic powered the Belarusian team to a surprise victory. Roma staged a second-half comeback with goals from Gervinho and Vasilis Torosidis, but was denied a point when Alessandro Florenzi's shot hit the crossbar late on.
Barca rose to the top of the group with four points, followed by Leverkusen and BATE Borisov with three each. Roma is last with one point.
Group F
A goalkeeping blunder by David Ospina helped Olympiacos beat Arsenal 3-2, leaving the Gunners in real danger of its first group-stage exit in 16 years.
Ospina fumbled a corner over his line to give the visitors a 2-1 first-half lead, and substitute Alfred Finnbogason scored the winner in the 66th minute – almost immediately after Alexis Sanchez had drawn Arsenal level.
Arsenal, which has advanced past the first group stage every year since 2001, has now lost both its opening games – and still has two meetings with Group F leader Bayern Munich to come. Bayern beat Dinamo Zagreb 5-0 to take a tight grip on first place in the group.
"It leaves us in a bad position, but we are still in it," Arsenal manager Arsène Wenger said. "We have to make a result in our next game against Bayern at home.
"We are not out of it."
Wenger rested first-choice goalkeeper Petr Cech, just as in the opening 2-1 loss at Dinamo.
In Munich, Dinamo never came close to repeating that victory in a one-sided game. Bayern was up by four goals after 28 minutes in a comfortable win that saw a hat trick for Robert Lewandowski and goals for Douglas Costa and Mario Gotze.
Group G
Chelsea's miserable start to the season opened a new chapter as it lost top spot in Group G with a 2-1 defeat at FC Porto.
Jose Mourinho's former club went ahead in the 39th, when Brahimi beat Branislav Ivanovic and Gary Cahill on the left, with goalkeeper Asmir Begovic pushing away Brahimi's shot only as far as Andre Andre, who fired a first-time volley into the net.
Willian levelled with a curling free-kick in first-half added time as Iker Casillas, making a competition record 152nd appearance, didn't even move.
Maicon sealed it in the 52nd, putting the Portuguese club level with group leader Dynamo Kyiv on four points, one ahead of Chelsea, which is also struggling in the Premier League.
In Israel, Dynamo cruised to a 2-0 win over Maccabi Tel Aviv, which is yet to score a goal in this season's group stage. Andriy Yarmolenko put the Ukrainian team ahead in the fourth minute, before Junior Moraes struck five minutes into the second half.
Group H
Hulk delivered a man-of-the-match performance as Zenit Saint Petersburg made it two wins from two matches in Group H with a 2-1 defeat of KAA Gent.
Hulk had two assists, while Artem Dzyuba scored his first Champions League goal and Oleg Shatov notched the winner.
Meanwhile, Valencia got its campaign back on track with a 1-0 win at Olympique Lyonnais following an opening home loss to Zenit.
Midfielder Thomas Matton scored his fourth goal in three games for Gent, equalizing in the 56th minute, but Shatov ended Gent's 12-game unbeaten start to the season.
Zenit remains on top of Group H with six points, three more than Valencia. Lyon and Gent have one point.
In Lyon, Sofiane Feghouli's powerful strike in the 42nd minute was left unanswered despite the hosts' heavy pressure in the second half. But a lack of cutting edge and two excellent saves from Valencia 'keeper Jaume Domenech sealed Lyon's fate.
Lyon could have taken the lead in the 27th minute, when Mathieu Valbuena's curled free kick forced Domenech to tip the ball onto his post. Valencia also hit the woodwork twice before the interval.