Tuesday's Champions League match between Barcelona and Juventus will feature the first meeting between Lionel Messi and Cristiano Ronaldo since the Portugal forward left Real Madrid in 2018 to join the Italian champs.

Ronaldo missed the first match between the two teams in Turin -- a 2-0 Barca win -- after contracting the coronavirus but is available again for this week's fixture at Camp Nou.

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Messi, meanwhile, has been rested for Barca's past two Champions League games with qualification for the round of 16 already secured, but Ronald Koeman said he will return to the squad to face Juve.

"Leo and [Ronaldo] are two fantastic players who've been great to watch for so long," the Barcelona coach said in his pregame news conference.

"They are the best of the last 15 years. They have achieved so much success. They are different players but I admire them both. They have given us many great nights and I hope we enjoy them both [on Tuesday]."

Messi and Ronaldo have marked an era in European football, with one or the other winning the Ballon d'Or in 11 of the past 12 years.

Their rivalry came to define the Clasico between Barca and Madrid over the past decade, with their performances and goals driving their respective clubs to success both domestically and in Europe.

They are both now in the latter stages of their careers -- Messi is 33 and Ronaldo is 35 -- but remain among the best players in the world.

"Ronaldo's still among the elite because of how he fights and the goals he scores," Koeman said.

The match between Barca and Juve will decide who tops the group, with Koeman's side three points clear in top spot, but things are not going as smoothly for them in La Liga.

Saturday's 2-1 loss at Cadiz was their fourth in 10 league games this season, corresponding to their worst start to a campaign since the 1987-88 season.

"I am unhappy about the situation in the league," said Koeman, who criticised the players' attitude at the weekend. "There are differences between the Cadiz loss and the Atletico [Madrid defeat two weeks ago].

"If I lose, I am angry, and that's what I told the players after the game. You cannot accept the goals we have conceded several times now at a club like Barcelona.

"The message for the players is to remind them of the responsibility they have being here. What is said specifically in the dressing room stays there, but we have to be more effective and make fewer mistakes."

Messi has scored just twice from open play in 13 appearances for Barca this season and has been the subject of criticism following their latest defeat.

However, Koeman said he's an easy target because of his status and defended the forward, who's free to talk to other clubs from January onwards with his contract expiring in June.

"Leo's important to our attack -- the most important part, in fact," he said. "Most moves come from his feet. Sometimes it's easiest to blame Messi, because he's the best in the world."