Cristiano Ronaldo is back in contention as Manchester United begin the defence of their European crown on Wednesday when they take on Spanish hopes Villareal at Old Trafford.

The Portuguese winger has recovered from ankle surgery and could make a timely return as United aim to become the first club to retain the Champions League title.

Ronaldo has been missed in an indifferent start to the season for United, who have to pick themselves up for the Group E game following a 2-1 defeat at Liverpool on Saturday.

Meanwhile, UEFA Cup winners Zenit St Petersburg - who defeated United in the Super Cup last month - test their Champions League mettle with a trip to Italian giants Juventus.

Elsewhere, four-time European champions Bayern Munich, Germany's only contender, are back in Champions League action after a year's absence with a visit to Romania's Steaua Bucharest.

United manager Sir Alex Ferguson is looking for a quick reaction from his players after the Anfield defeat. The Villareal test is followed by a Premier League visit Sunday to Chelsea, the team United beat on penalties in last season's Champions League final in Moscow.

"You have to recover, that's the name of the game. It's the Champions League and we are in a good group and I think we have a good chance of qualifying," Ferguson said.

"The way I look at the Champions League is to get to 10 points. If you get that you know you have qualified and you can look forward to February when it really starts to kick in."

AC Milan were the last side to retain the European Cup in 1990 but no team has won back-to-back titles since the Champions League was launched in 1992/93.

"The fact that no club has won it in successive seasons shows just how hard it is to win the thing in the first place. But I really do think we've got a great chance," Ferguson said.

Ronaldo may feature after making a quicker-than-expected recovery from ankle surgery, United said on its website. But Michael Carrick is likely to miss the match after injuring his right ankle at Anfield, while fellow midfielder Paul Scholes is suspended.

Villarreal, who go into the game with a weekend 1-0 win over Deportivo, have former Arsenal winger Robert Pires looking forward to a return to playing on an English ground.

In the other group match, Scottish champions Celtic take on Danish side Aaalborg.

Group H sees an intriguing clash when Juventus take on Russian champions Zenit. The Turin side have had had a good start in the Italian Serie A but Zenit, who again impressed in defeating United 2-1 for the Super Cup in Monaco on August 29, are highly fancied.

Veteran captain Alessandro Del Piero and Frenchman David Trezeguet are expected to return for the Italians after being rested for Sunday's 1-0 victory over Udinese.

In the same group, nine-time European champions Real Madrid face unfancied Bate Borisov, the Belarussian side who are making their debut in the Champions League.

The Spanish champions are sweating on the fitness of right-back Sergio Ramos, while defender Pepe is suspended. However, veterans Fabio Cannavaro and Gabriel Heinze are back to fitness, while all-time Champions League top scorer Raul Gonzalez is also set to return after being rested in Sunday's 4-3 win over Numancia.

Juergen Klinsmann has left France midfielder Franck Ribery at home for the trip to Steaua Bucharest as Bayern Munich return to the Champions League stage after an unwelcome season in the UEFA Cup.

Ribery, who underwent ankle surgery following Euro 2008, is back in training but Klinsmann said the midfielder is not quite ready.

Bayern are "extremely motivated" to make an impact on the competition, chairman Karl-Heinz Rummenigge said.

"It is important we give a signal after a one-year absence and immediately establish a firm foundation," he said.

Group opponents Fiorentina, meanwhile, rejoin the Champions League after an eight-year absence, beginning with a trip to French champions Olympique Lyon, who have played the past nine editions.

Group G sees 2004 Champions League winners Porto take on Fenerbahce, while Premier League side Arsenal visit Dynamo Kiev.