Liverpool begins the defense of its Champions League title with a return to Serie A side Napoli in an enticing opening round of matches in the Champions League this week.

Spanish big-guns Barcelona, Real Madrid and Atletico Madrid are meanwhile involved in the pick of the games today and tomorrow as 32 teams begin the campaign in Europe's most prestigious and lucrative club competition.

Barcelona visits German runner-up Borussia Dortmund today in a first Champions League meeting between the two sides, but home fans may not get the opportunity of seeing Lionel Messi.

The Argentina forward and Barca captain trained with teammates on Sunday as he recovers from a preseason calf injury, but both he and Ousmane Dembele are expected to miss the Group F game.

Messi, however, is looking forward to the visit to the Bundesliga side. "The competition is fairly even and only the best make it here. That's what makes this tournament is so special. We got Dortmund, whom we've never played [in the Champions League]. I remember seeing its stadium on TV and I've always liked it because of the fans and what the club itself represents," he told UEFA.com.

Barcelona could field 16-year-old Ansu Fati after he scored on his full debut for them at the weekend. "It's not normal that with his first touch he scores a goal," said coach Ernesto Valverde after Fati netted in two minutes and then set up the second on six minutes as Barcelona beat Valencia 5-2.

Record European champions Real Madrid travels tomorrow to big-spending Paris Saint-German, a team it beat on the way to its Champions League titles in 2015-16 and 2017-18.

Real coach Zinedine Zidane can welcome back Gareth Bale, who was suspended on the weekend and could give a Madrid Champions League debut to Eden Hazard. His team meanwhile comes up against keeper stalwart Keylor Navas, who swapped the Bernabeu for the Parc des Princes in the summer, in the Group A showdown.

Atletico Madrid has a Group D home encounter with Italian champions Juventus and Cristiano Ronaldo, who scored a hat trick to overturn a first-leg 2-0 deficit when the two sides met in last season's last 16.

Liverpool, now five points clear at the top of the Premier League, will be without injured striker Divock Origi in its Group E opener at Carlo Ancelotti's Napoli, a team it lost to 1-0 on its last trip there last season. It was only a last-gasp save by keeper Alisson against the Italians on the last match day which gave Liverpool a 1-0 win at Anfield to progress at Napoli's expense.

After last season's all-England final in June, and four teams in the quarterfinals, the Premier League contingent will expect another strong challenge. Champions Manchester City is at Shakhtar Donetsk (Group C), Chelsea hosts Valencia (H), and last season's losing finalists Tottenham Hotspur visits Olympiakos (B).

The Bundesliga will be hoping for a better showing than last season when no side made the last eight, and ambitious Bayern is aiming for a good start at home in Tottenham's group against Red Star Belgrade tomorrow.

Bayer Leverkusen meanwhile greets Lokomotiv Moscow in Group D tomorrow, and RB Leipzig has a Group G visit to Benfica today with confidence high following a 1-1 draw with Bayern which kept them top of the Bundesliga. "It's generally very important to start internationally with a feeling of stability from the league," coach Julian Nagelsmann said. "This good feeling with which we can now travel to Lisbon will help us play well against Benfica."

Last season's losing semifinalists Ajax begins with a home game against OSC Lille in Group H today, and Champions League newcomer Atalanta is at Dinamo Zagreb in Group C tomorrow.

The other games are: Club Brugge vs. Galatasaray (Group A), Salzburg vs. Genk (E), Inter Milan vs. Slavia Prague (F), and Lyon vs. Zenit St Petersburg (G).