MADRID — Barcelona's first game without the injured Lionel Messi will be an important one in the Champions League.
The defending champions face Bayer Leverkusen, the Group E leader, in the second round of group stage matches on Tuesday.
After a 1-1 draw with Roma, a setback at home could leave the Spanish club in an uncomfortable position early on in the group and be a worrisome sign about the future without its biggest star.
Messi will be sidelined for nearly two months after tearing a ligament in his left knee in a Spanish league match on Saturday.
"I'm sad anytime a player gets injured, and if it is Messi even more so, considering the weight he has on the team," Barcelona coach Luis Enrique said. "Whenever there is an injury, the important thing is the team. We are a team and will pull through this together."
AS Roma plays BATE Borisov in the other game in Group E.
Real Madrid will be looking for a second consecutive victory in Group A when it plays at Malmo on Wednesday, while its group rival, Paris Saint-Germain, can also reach six points from two matches when it plays at Shakhtar Donetsk.
In a game between Group C leaders, Atletico Madrid hosts Benfica on Wednesday, and Zenit St. Petersburg can maintain its early lead in Group H when it hosts Belgium's Gent on Tuesday.
Here are other things to look for this week in the Champions League:
English teams Arsenal, Manchester City and Manchester United opened the group stage with losses and need to get back on track this week.
Manchester United, the Premier League leader, hosts Wolfsburg in Group B, while Arsenal plays Olympiakos in Group F. Manchester City, with three losses in its last four games in all competitions, is away at Borussia Moenchengladbach in Group D.
"Being a strong team is Europe must be our target," City coach Manuel Pellegrini said. "But not at the expense of the Premier League."
Chelsea won its opener in Group G, but Jose Mourinho's team travels to Portugal for a difficult game against FC Porto, where his managerial career took off by winning the Champions League in 2004.
Juventus captain Gianluigi Buffon believes a victory against Sevilla in Group D on Wednesday could help the team turn its season around after a difficult start at home.
"The Champions League gives us an excellent opportunity to rediscover our confidence and spark," Buffon said. "We need to face Sevilla with ferocity, determination and a healthy desire to surprise."
The four-time defending Italian league champions lost for the third time this season over the weekend, falling 2-1 at Napoli.
"This is the toughest challenge we've faced," Buffon said. "But I live for challenges and the more difficult they are, the greater the joy of overcoming them."
Borussia Moenchengladbach's renaissance is a timely one.
Lucien Favre quit as coach last week after starting the Bundesliga with five defeats and a 3-0 loss to Sevilla in the sides' opening Champions League game. However, interim coach Andre Schubert has now led the team to two wins, with seven goals scored.
Ahead of the club's first European Cup game at home for 37 years, 'Gladbach finally appears to have clicked.
"We're playing the football that made us strong last season again," midfielder Granit Xhaka said after Saturday's 3-1 win at Stuttgart. "We're very happy that it's going well again because the next big challenge is waiting for us."
Kazakhstan will be hosting its first-ever match in the Champions League proper on Wednesday.
Since Kazakhstan is geographically considered part of Central Asia and in football terms only switched from Asia to Europe in 2002, FC Astana's participation in the tournament has raised some eyebrows.
The match against Galatasaray in Kazakhstan's capital, Astana, has brought excitement to a country where international football talent rarely visits, outside of the occasional World Cup or European Championship qualifier, although expectations are being kept firmly in check.
Astana coach Stanimir Stoilov may be particularly keen for a win, having spent part of his playing career in Turkey with Galatasaray's fierce rival Fenerbahce.
AP Sports writers Rob Harris in Zurich, Daniella Matar in Milan and Ciaran Fahey in Berlin contributed to this report.
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