5 things to know about the Champions League

LONDON (AP) Half of the last 16 places in the knockout stage of the Champions League are still up for grabs and will be decided this week after the final group games. Manchester United, Real Madrid, Chelsea and Paris Saint-Germain reached the knockout phase in the last round of games, joining defending champion Bayern Munich, Manchester City, Atletico Madrid, and Barcelona.

Among the big clubs, Arsenal will clinch qualification and top spot of Group F with a win or a draw at Napoli while AC Milan needs a draw at home against Ajax in Group H. After a shaky start to its campaign, Juventus will be through with a draw at Galatasaray in Group B. Here are five things to know about this week's matches in Europe's top competition:

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ARSENAL'S TOUGH SCHEDULE

Arsenal may have 12 points in Group F, but the Gunners have yet to qualify for the knockout stage. Napoli and Dortmund are both three points behind Arsene Wenger's team, which only needs a draw to be guaranteed a spot in the round of 16.

Arsenal has also been the dominant force in the Premier League so far this season but Wenger is now concerned his players' tight schedule might took its toll. After playing Everton on Sunday afternoon, the Gunners visit Napoli on Wednesday before a trip at Manchester City three days later.

"I would like that there is an ethics committee who looks if the clubs are in the right position and have the right protection for English football to do well," Wenger said. "It is a fact the way football is sold that every television (station) has its own rights. They don't care when you have already played. They think: 'Is it a nice time to play on Saturday morning this game between Arsenal and Man City? OK.' They don't look to see if we played on Wednesday. The way football is sold does not protect you."

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IN A MESS WITHOUT MESSI

Without injured forward Lionel Messi and goalkeeper Victor Valdes, Barcelona has lost back-to-back games for the first time since March and now finds itself needing a point against Celtic to ensure it finishes first in Group H on Wednesday.

Barcelona rallied for a 4-1 win at third-tier Cartagena on Friday in its Copa del Rey opener, but doubts still linger over whether Gerardo Martino's side can recover its dominant form without Messi, following defeats at Ajax in the Champions League and Athletic Bilbao in Spain.

Barcelona may only need a draw, but anything less than a victory would hardly be tolerated by the always demanding Camp Nou crowd.

"We have analyzed what we did wrong and now we can look forward to how we can improve," said Barcelona midfielder Alex Song. "We want to win the game (vs. Celtic) and have a good performance. We have a lot to show after the last game in the Champions League."

Barcelona will be hoping to recover midfielders Andres Iniesta and Xavi Hernandez after both were rested on Friday to heal niggling muscle injuries.

AC Milan hosts Ajax on Wednesday also needing a draw to finish the group in second place and book a spot in the knockout rounds. But a third setback in four games for Barcelona would allow the Italian team to claim the group with a victory.

Real Madrid and Atletico Madrid, meanwhile, already have first place in their respective groups locked up. That means both sides may opt to give playing time to their promising youth players like they did on Saturday in the Copa del Rey when Real started Jese Rodriguez and Alvaro Morata and Atletico started Oliver Torres and Javier Manquillo.

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DORTMUND'S INJURY BLIGHT

The good news for Borussia Dortmund is that a win at Marseille would guarantee progress to the second round because of the side's superior head-to-head record against both Arsenal and Napoli.

The bad news for coach Juergen Klopp, however, is that his side has been decimated by injuries, with Nuri Sahin and Sven Bender the latest to be likely ruled out of the game at the Stade Velodrome. Both midfielders hobbled off with ankle injuries during Saturday's 1-0 home defeat to Bayer Leverkusen.

"Miracles happen again and again but not so often," Klopp said about his hopes for the players' speedy recoveries.

Sahin and Bender join first-choice defenders Mats Hummels, Neven Subotic and Marcel Schmelzer on Dortmund's injury list, together with midfielder Ilkay Guendogan, giving Klopp a conundrum for Wednesday. Defender Manuel Friedrich, signed at short notice to cover the injuries, is not eligible for the Champions League.

Marseille is bottom of the group without a point with problems of its own. The French side fired coach Elie Baup on Saturday after losing 1-0 to Nantes the day before.

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BALOTELLI'S BACK

After struggling with his form, temper and injuries, Mario Balotelli has rediscovered his scoring touch.

The Italy striker scored both goals in AC Milan's 2-2 draw at Livorno on Saturday and also hit the post, giving him four goals in the Rossoneri's last three matches a sharp turnaround after a six-match scoreless streak in all competitions.

"I've always been fine," Balotelli said. "There were never any big problems like was written."

Also at Milan, Kaka continues to show solid form after rejoining the club from Real Madrid he set up Balotelli's opener vs. Livorno but the Rossoneri defense has been problematic.

"We need to be more attentive when our opponents go on counterattacks," Balotelli said.

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DIFFERENT PRIORITIES

Paris Saint-Germain has nothing to gain from its last group game. The French club will finish top of Group C whatever the result of its match against Benfica on Tuesday.

PSG coach Laurent Blanc is now more focused on the French league and said he won't play Sweden striker Zlatan Ibrahimovic in Lisbon.

"The French league is our goal for the end of this year since we got the job done in the Champions League," Blanc said. "I'll try to start the best lineup for the games in the French league in order to take points. I can already tell you that Ibra will skip the Lisbon match."

Ibrahimovic has proved instrumental in PSG's success this season, netting eight goals in the lucrative European competition to be the top scorer, tied with Real Madrid striker Cristiano Ronaldo.

PSG leads the French league but is under pressure from Lille and Monaco.