A fit and firing Cristiano Ronaldo is "really happy" now his knee problem is behind him, according to Juventus boss Maurizio Sarri.

Portugal international Ronaldo started 2020 with a bang on Monday as he netted a hat-trick in the 4-0 win over Cagliari to take his tally to eight goals in his past five Serie A appearances.

The 34-year-old appears to have rediscovered his mojo having gone three games without scoring at the start of November, when he was twice substituted by Sarri as he battled a minor knee problem.

There has been no sign of that issue affecting Ronaldo of late, though, with Sarri delighted the five-time Ballon d'Or winner is back to his best.

"Ronaldo has not any more problems with his knee and now he is enjoying his great physical and mental condition," the Juventus head coach told a news conference.

"Finally I can see him really happy because his knee is fine now. He can play at his best now."

Another Bianconeri player who has been troubled by injury issues is defender Matthijs de Ligt.

The former Ajax captain was an unused substitute in the victory against Cagliari having not started a league game in a month due to shoulder and adductor problems.

De Ligt has yet to replicate the type of form which earned him the move to Turin ahead of this season, yet Sarri is convinced he can fulfil his promise.

"The situation of De Ligt is very clear: I'm sure he'll become one of the best - or the best maybe - defenders in the world," Sarri added.

"We need to understand that he was just 19 coming to a club like Juventus and forced to play 23 games in a row.

"He needs to learn a new language and new training technique. In Italy he had to change his way to play and his lifestyle. He had a lot of work to do in the last months.

"He improved a lot, then he had some little injuries. But that's normal."