Liverpool travel to Napoli, where they lost in the group stage last season, while Barcelona visit Borussia Dortmund as the Spanish giants renew their quest for a first European title since 2015.

Last season's surprise semi-finalists Ajax are also in action, against Lille at home, while Europa League champions Chelsea welcome Valencia to Stamford Bridge.

In Group E, Liverpool know from experience that they can't take anything for granted in Naples.

"It is a really tricky place to go. The fans are really hostile," fullback Trent Alexander-Arnold said.

Forward Divock Origi is out after twisting his ankle at the weekend while midfielder Naby Keita and goalkeeper Alisson Becker remain sidelined.

Mexico winger Hirving Lozano and veteran striker Fernando Llorente, who made his debut last weekend off the bench, give Napoli coach Carlo Ancelotti more attacking options this season.

"Llorente is a complete forward. Because he is so tall, everyone thinks he's just an old-fashioned centre-forward, but he can play football and has great technique," Ancelotti said.

Austrian champions Salzburg host Belgian counterparts Genk in the other Group E game.

Dortmund fans are hoping Spanish forward Paco Alcacer can keep his scoring streak going when they host his former side Barcelona in Group F.

Alcacer has scored in every competitive game so far this season - seven goals in six games for Dortmund and three in two games for Spain.

Barcelona star Lionel Messi could feature as his recovery from a lingering calf injury gathers pace.

"We will decide (if Messi plays) tomorrow," Barcelona coach Ernesto Valverde told a news conference Monday.

Messi trained with his teammates on Sunday and Monday but has yet to play in the league this campaign after being injured in the pre-season.

Striker Ansu Fati, 16, has been included in Barca's squad and may make his Champions League debut after two goals in three league appearances.

Slavia Prague's daunting task to emerge from the group begins at Inter Milan, who lead Serie A after three wins from as many games.

Lyon open their campaign at home to Russian champions Zenit St. Petersburg, widely considered the weakest of the competition's eight top seeds.

Benfica host German side Leipzig in the other Group G game with all four teams fancying their chances of reaching the knockout phase.

Valencia arrive at Group H favourites Chelsea in crisis after last week's firing of coach Marcelino and a 5-2 drubbing at Barcelona in Albert Celades' first game in charge.

Conversely, Frank Lampard begins his bid to win the Champions League as manager after lifting the trophy as a Chelsea player - with the club's academy graduates Mason Mount, Tammy Abraham and Fikayo Tomori firing.

"It is the top club competition in world football and I'm sure Mason and Tammy and Tomori are all excited for it," Lampard said.

Meanwhile, Lille coach Christophe Galtier is under no illusions about the task facing his side at Ajax - despite the Dutch club's sale of centre back Matthijs de Ligt and midfielder Frenkie de Jong in the off-season.

"We know how good a side they are and how difficult it will be," Galtier said.

Australian Associated Press