The Champions League may only be heading into its second round of matches this week, yet Chelsea and Borussia Dortmund are already under pressure to claim victories in Europe's top club competition.

Most of the continent's top teams got off to flying starts two weeks ago, with Barcelona, Real Madrid, Bayern Munich and Manchester United scoring 17 goals between them to take early command of their groups.

However, 2012 winner Chelsea slipped up 2-1 at home to Basel and Dortmund - last season's beaten finalist - lost by the same score line at Napoli. Both will need to get their campaigns back on track this week.

Here are 5 things to know about the upcoming round of Champions League matches:



Last season Chelsea became the first defending champion to fail to get out of the group stage, but was expecting to fare much better this time round with the return of two-time Champions League-winning coach Jose Mourinho. Opening up with a home defeat to Basel in Group E certainly wasn't in the script and the English team now faces a trip on Tuesday to Romania's Steaua Bucharest, where it lost 1-0 in the Europa League last 16 last season. At least that group appears to give Chelsea some leeway to recover. Dortmund, on the other hand, can't afford another slip-up at home to Marseille on Tuesday, with Arsenal and Napoli - who are first and second in England and Italy respectively - the other teams in Group F.



Tears of joy streamed down Rod Stewart's cheeks the last time his beloved Celtic hosted Barcelona in a Champions League game. On that memorable night last November, the Scottish champions produced one of the most stunning results in the competition's history to beat Barca 2-1 and reduce rock star Stewart to an emotional wreck. The following day, another pop icon, Elton John, phoned Celtic manager Neil Lennon to offer his congratulations. Can history repeat itself on Tuesday when Celtic hosts Barcelona in the Group H match? Well, Celtic lost 1-0 at home to second-tier Morton in the League Cup last week and Barcelona has not only made its best ever start to a La Liga campaign with seven straight wins but also thrashed Ajax 4-0 in its Champions League opener. Lionel Messi will be missing for the visitors with an injury to his right leg, offering Celtic renewed hope of another famous upset.



Manchester United manager David Moyes may be glad to see the back of the Premier League for a few days. After making its worst six-game start to a domestic season in 24 years, United is languishing in 12th place in the standings and is already eight points behind early leader Arsenal. The transition from Alex Ferguson to Moyes is proving tough to swallow for United fans, who won't have been too thrilled to hear their new coach play down the team's chances in Europe this season. ''To win the Champions League, you need five or six world-class players,'' Moyes said. ''Look at Bayern Munich, they have it. Look at Barcelona, who had it in the past and Real Madrid, who have maybe got it now. That's the level you have to be at to win it. We've not got that yet but what we have got is experience.'' United started with a 4-2 home win over Bayer Leverkusen in Group A and travel to Ukraine's Shakhtar Donetsk on Wednesday.



It's comfortably the standout game of matchday two and this Group D showdown gives Bayern a first real test in its defense of the trophy. Lacking recent experience in Europe, City failed to advance from its group over the last two seasons but began this campaign well with a 3-0 win at Viktoria Plzen 3-0. It also has a wily new coach in Manuel Pellegrini, who exceeded expectations in the Champions League with Villarreal and Malaga in the past. Bayern easily beat CSKA Moscow 3-0 in its first group game and is now 32 matches unbeaten in the Bundesliga after a labored 1-0 win over Wolfsburg on Saturday.



The group stage began in record-breaking fashion with 30 goals being scored in eight matches on the opening night, including Real Madrid smashing six past Galatasaray and Paris Saint-Germain putting four by Olympiakos. That demolished the existing goals-per-game record for an opening matchday and, with 23 more goals scored on the Wednesday, underlined the strength and depth of attacking talent available to Europe's heavyweight teams. Madrid failed to score in a rare derby defeat to city rival Atletico on Saturday but should be back in the goals at home to FC Copenhagen in Group B on Wednesday, with Cristiano Ronaldo looking to add to his hat trick against Galatasaray.