Hamburg - Holders Manchester United will be out to kick- start their Champions League title defence when they travel to Denmark to face AaB Aalborg on Tuesday.

Having drawn at home with Villarreal in their opening game of the group stages, United will be looking for three points from the Group E encounter, and will have fit-again Cristiano Ronaldo available to start.

Also in Group E, Spanish league leaders Villarreal will be at home to Celtic. Around 5,000 Celtic fans are expected to make the trip, and local police will be working hard to avoid problems.

Villarreal's only injury victim is striker Nihat Kahveci, badly injured playing for Turkey at Euro 2008 in June.

German champions Bayern Munich, meanwhile, will be hoping to put two successive league defeats behind them, the latest a 1-0 loss to Hanover, by building on the opening round win over Steau Bucharest with victory over Olympique Lyon.

Bayern have made their worst start to a Bundesliga season in 31 years, winning just two of their opening six games, and new coach Jurgen Klinsmann is already under pressure.

Arsenal also suffered a shock defeat at the weekend, going down 2- 1 at home to Hull City, and the Gunners face the trickiest tie of the four British clubs in action when they host former champions Porto in Group G.

"This group is tougher than in previous years. It is definitely the most even group we have had because there are no easy games," manager Arsene Wenger said.

"That makes the match with Porto more important. We know in the Champions League you qualify with 10 points, so if we win our home games we are through."

Real Madrid are preparing for Tuesday's Champions League clash away to Zenit St Petersburg in good heart, despite their midfield injury problems.

The Spanish champions will make the trip without injured midfielders Fernando Gago, Wesley Sneijder and Guti - but in good spirits, thanks to Saturday's late win away to Betis.

Real won 2-1 in Seville thanks to an injury-time strike from the indefatigable Ruud Van Nistelrooy.

Real are joint top of Group H with Juventus after their opening 2- 0 win over BATE Borisov. Juventus visit BATE looking to maintain their 100-per-cent start in the Champions League.

Chelsea, beaten by United in the final in May but top of the Premier League after a good start, are also looking to make it two wins out of two when they go to Romania Wednesday to face CFR Cluj- Napoca.

The Blues will be without the injured Deco, Ricardo Carvalho and Michael Essien, but new assistant manager Ray Wilkins believes the experience of last year is making Chelsea more dangerous this time.

"The first day I walked in I could see the determination," Wilkins said. "Chelsea are a bunch of winners but were second in the Premier League, lost the Champions League final and lost the final of the Carling Cup. No wonder they're smarting."

Italian champions Inter Milan suffered their first defeat this season losing 1-0 to AC Milan, but appear ready to bounce back against visiting Werder Bremen in Group B.

Captain Javier Zanetti, who at the weekend played his 601st game with the blue-blacks said that "Milan won the derby, but we look ahead. Now we must try and win in the Champions League."

Also on Wednesday, Barcelona will be away to Shakhtar Donetsk in Group C action.

Barca coach Josep Guardiola will be tempted to ring the changes after Saturday's exhausting 2-1 derby win over Espanyol.

The candidates to come in are Rafa Marquez, Martin Caceres, Yaya Toure and Bojan Krkic.

Five-times winners Liverpool, who won in Marseille in their opening match, take on PSV Eindhoven at Anfield, hot on the heels of victory over Everton at the weekend which moved them alongside Chelsea at the top of the league.

Captain Steven Gerrard, though, has warned his teammates to focus only on the next match.

"It's important to be humble and not get carried away by the performance," he said.

"But we set a standard (at Everton) we need to match week in week out."

In the other group game, Atletico Madrid are at home to Olympique Marseille.