Barcelona and AC Milan made winning starts to a Champions League group containing four clubs who have all won European football's biggest prize, but Celtic and Ajax still have reason to be hopeful heading into the second round of matches on Tuesday.

Here are five things to know about the Celtic-Barcelona and Ajax-AC Milan matches in Group H:

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ANOTHER TEARFUL NIGHT FOR ROD STEWART?

It was one of the most remarkable results in Champions League history, and even reduced rock star Rod Stewart to tears of joy in the crowd. Celtic beat Lionel Messi and the rest of Barcelona's superstars 2-1 at Celtic Park in the group stage last season, weeks after narrowly losing 2-1 at Camp Nou after a late goal by Jordi Alba. Barca's record of just two wins from six matches in Scotland suggests this will be no pushover, despite the huge gulf in class in the squads. ''I think it will be very unprofessional of them if they didn't think that we were a threat,'' Celtic playmaker Kris Commons said.

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NO MESSI

Celtic's chances of causing another upset improved when Messi was ruled out of the match because of a right thigh injury that will sideline the Argentina forward for two to three weeks. ''The team is completely different with him,'' Barcelona defender Adriano said. Messi was hurt during a 2-0 win at Almeria on Saturday, completing a tough couple of days for the world's best player. On Friday, he appeared with his father before a judge to answer questions in a multimillion-euro tax fraud case. Summer signing Neymar was kept on the bench at Almeria, so should be well-rested for the trip to Scotland.

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NO CONTEST - ON PAPER

On paper, there can only be one winner in Glasgow. While Barcelona has won its first seven matches to make its best ever start to a La Liga campaign in 82 seasons, Celtic is only second in the weak Scottish league and even lost 1-0 at home to second-tier Morton in the League Cup last week. Many Barcelona fans shrugged their shoulders when the club announced it had hired little-known Argentine Gerardo Martino to replace Tito Vilanova as coach this summer but he has already written his name in the history books. Five other coaches, including Pep Guardiola (2009-10) and Vilanova (2012-13), had six wins in a row to start La Liga.

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BALOTELLI BACK FOR MILAN

In the middle of a three-match domestic ban, maverick Italy international Mario Balotelli is available for AC Milan in Amsterdam but fellow striker Stephan El Shaarawy will miss out after fracturing a bone in his left foot in training on Monday. El Shaarawy, who had only returned from a month out with a muscle problem, expressed his disappointment on Twitter, saying: ''I don't believe it!'' Mattia De Sciglio, Kaka, Giampaolo Pazzini and Matias Silvestre are also out injured for Milan, which has side started the season poorly. Its 2-0 win over Celtic two weeks ago only secured through goals in the final eight minutes.

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AJAX LOOKING TO MAKE AMENDS

Ajax endured a tough start to the group, losing 4-0 at Barca and becoming the latest team to be run ragged by Messi as he scored a hat trick at Camp Nou. The Dutch side will also be the underdog against Milan, too, with Kolbeinn Sigthorsson saying the Italians are ''naturally a level - or two - higher as a competitor.'' But Ajax come into the match following a 6-0 win over Go Ahead Eagles on Saturday and took four points off Milan in the 2010-11 group stage. Striker Bojan Krkic will be missing with a hamstring injury.