Chelsea boss Jose Mourinho believes Tuesday's Champions League trip to Steaua Bucharest is a match they cannot lose while Premier League leaders Arsenal take on Napoli for the first time.

AFP Sports looks at all eight of Tuesday's fixtures on matchday two:

Group E

Basel (SUI) v Schalke (GER)

Basel, who made the Europa League semi-finals last season, stunned 2012 champions Chelsea 2-1 at Stamford Bridge in their opener with goals from Mohamed Salah and Marco Streller. It was the first win for a Swiss club on English soil in 20 attempts.

Basel lead the Swiss league but were held to a 2-2 draw by Sion.

The last time Basel faced a German side they lost 7-0 to Bayern Munich in a last-16 second leg tie in 2011-2012.

Schalke were also winners on matchday one, beating Steaua Bucharest 3-0. But the German side are only mid-table in the Bundesliga and on Saturday surrendered a 2-0 lead against Hoffenheim to draw 3-3. Kevin-Prince Boateng, man of the match against the Romanians, was on the scoresheet on Saturday.

Steaua Bucharest (ROM) v Chelsea (ENG)

Chelsea warmed-up for Tuesday's tie by drawing 1-1 at London rivals Spurs in a match which saw Fernando Torres sent-off and John Terry score his team's only goal. Juan Mata will start the match that coach Jose Mourinho says is a game Chelsea can't afford to lose.

Steaua beat Chelsea 1-0 last season in the Europa League round of 16 first leg. Mata levelled the overall scores at Stamford Bridge before Vlad Chiriches restored Steaua's aggregate lead. But Terry and Torres struck to take the Londoners through.

The Romanians are playing in the group stage for the first time since the 2008-2009 season. Their last victory came at Dinamo Kiev in September, 2006.

Group F

Arsenal v Napoli

The Gunners' remarkable turnaround after an opening day defeat to Aston Villa in the Premier League has seen them reel off nine consecutive wins, including a 2-1 victory away to Marseille in their Group F curtain raiser.

Arsene Wenger celebrated his 17th anniversary as Arsenal boss with a 2-1 win away to Swansea over the weekend with in-form midfielder Aaron Ramsey on target for a fourth game running. Theo Walcott is sidelined, but Czech midfielder Tomas Rosicky could return after a month out.

Rafael Benitez's Napoli are also off to a strong start in Italy's Serie A, having dropped just two points from a possible 18. Benitez will have a rested Marek Hamsik to call upon.

Borussia Dortmund v Marseille

Last season's finalists Borussia Dortmund are off to a flying start in the Bundesliga with six wins from seven matches, including a 5-0 thrashing of Freiburg on Saturday. However, Jurgen Klopp's men have work to do on the European stage after slumping to a 2-1 defeat at Napoli a fortnight ago. Goalkeeper Roman Weidenfeller, who was sent off in Italy, is banned while left-back Marcel Schmelzer is sidelined with a thigh problem. Marseille's first trip to Signal Iduna Park two seasons ago culminated with a dramatic late winner from Mathieu Valbuena as the visitors came from two goals down to win 3-2 and qualify for the round of 16.

Group G

Zenit Saint Petersbug v Austria Vienna

Both sides will be desperate to take all three points, having lost their openers to Atletico Madrid and FC Porto. Zenit, who lost 3-0 in Madrid, will be the more confident of the two, playing at home just a few days after going top of the Russian Premier League thanks to a 4-2 win over Spartak Moscow.

Alexander Kerzhakov became the all-time leading scorer in the Russian league in that game as he notched his 208th career goal.

Austria Vienna, 1-0 losers at home to FC Porto, went down 1-0 at Admira and are in fifth position in the top flight league. They are playing in the Champions League Group stages for the first time.

FC Porto v Atletico Madrid

Atletico have made a fantastic start to the campaign with six consecutive wins in all competitions, culminating in a first league victory over Real Madrid for 14 years on Saturday.

Unfortunately for Diego Simeone's men, talisman Diego Costa is unavailable as he serves the final match of his four-game European ban.

His absence is likely to mean a first ever Atletico start for Leo Baptistao, who scored in the Spanish side's 3-1 victory over Zenit St Petersburg two weeks ago.

Porto sit two points clear of Sporting Lisbon at the top of the Portuguese league after six games.

However, Paulo Fonseca's men will have bad memories of facing Spanish opposition after being dumped out at the quarter-final stage by Malaga last season.

Group H

Ajax v AC Milan

Ajax will be looking to bounce back from the 4-0 mauling they suffered at the hands of Barcelona and can hold out some hope against an Italian side that has won just twice in six Serie A games. Ajax warmed up for the match with a 6-0 thrashing of Go Ahead Eagles to leave them a point off leaders PSV Eindhoven. An injury-hit Milan will be seeking to secure their second successive win in the competition -- they beat Celtic 2-0 in their opener.

Celtic v Barcelona

Celtic gave Scottish football a rare moment of joy last season when they recorded an historic 2-1 win over Barcelona in the Champions League group stage. This time round Celtic manager Neil Lennon is not expecting a repeat.

However, Barcelona will be without Lionel Messi who suffered a thigh injury in Saturday's 2-0 win over Almeria. With his tough tackling compatriot Javier Mascherano also out of the match with a hamstring injury it poses some problems for coach Gerardo Martino. Defeat for Celtic would be their 100th in all European club competitions.