Cristiano Ronaldo delivered a damning assessment of Real Madrid’s defensive display against Napoli in the Champions League on Tuesday night, insisting at half time that his side “don’t know how to defend”.

Real secured their place in the last eight of the competition after beating the Italians 3-1, but found themselves a goal down heading into the break after Dries Mertens put the hosts ahead in the 24th minute.

Madrid’s defensive weaknesses were repeatedly exploited by Napoli’s early game plan, with the Italians’ high press causing numerous problems for Zinedine Zidane’s side.

During the half-time break, Madrid held a brief inquest in the tunnel before returning to the pitch during which Ronaldo was seen on live Spanish TV telling team-mate Pepe: “We don’t know how to defend.”

Real went on to address their poor first-half form and eventually put the hosts to bed, scoring three times in the second half to win the last-16 tie 6-2 on aggregate.

Captain Sergio Ramos netted one and had another header deflected in by Mertens to put Real in control with 30 minutes to go.

Alvaro Morata followed up Ronaldo's injury-time shot to complete a comfortable victory for the visitors.

Ronaldo's comments were put to goalkeeper Keylor Navas, who was involved in the half-time discussion, at the end of the game, with the Costa Rican admitting his side had been “disorganised” at times during the opening 45 minutes.

“We do the best we can, even though often we were left disorganised,” Navas said.

“But that does not make us defend badly. Often we defend well, cover spaces, people come across. There are many positive things and other things we can improve.”

Ronaldo in action for Real Madrid against Napoli (Getty)

Ramos admitted his side need to improve if they are to stand a chance of retaining their European title.

“I'm happy that I scored in a critical moment of the match, when we were suffering. But above all it was as a team that we got through,' Ramos said.