After getting off to winning starts in the Champions League, Barcelona and AC Milan face tricky away games in Group H.

Here are five things to know about the Celtic-Barcelona and Ajax-AC Milan matches on Tuesday:

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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. Scottish side Celtic beat Lionel Messi and the rest of Barcelona's superstars 2-1 at Celtic Park in the group stage last season. Barcelona even struggled at home against the Scottish champions, scoring late for a 2-1 win at Camp Nou. 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 striker Mario Balotelli is available for AC Milan in Amsterdam and could be partnered up front with fit-again Stephan El Shaarawy. It will give some potency to a Milan side that has started the season poorly, with its 2-0 win over Celtic two weeks ago only secured through goals in the final eight minutes. Mattia De Sciglio, Kaka, Giampaolo Pazzini and Matias Silvestre are still out injured.

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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.