In hindsight, Real Madrid's passage into the next round of the Champions League may have looked easy, with the side recording back-to-back 3-1 wins against tricky Italian side Napoli.But on both occasions they fell behind to the Serie A side and were forced to pick themselves up and fight back to ensure they came away with a positive result.Belgian forward Dries Mertens is currently having the season of his life, having notched up an impressive 23 goals across all competitions, and he put Real on the back foot when he connected with a superb through ball from teammate Marek Hamsik in the first half.And while it may have been a good goal by anyone's standards, it seems that Cristiano Ronaldo was far from happy with his defenders for allowing the 29-year-old through to score.So much so that cameras even caught Ronaldo giving his back line a stern talking to before they emerged back out of the tunnel for the start of the second half.With the players all ready to restart the action, he was picked up by the cameras telling teammates Marcelo, Pepe and Keylor Navas: "We do not know how to defend."None seem to know what to tell him with Marcelo appearing to be the only one willing to look him in the eye, giving him some sort of hand gesture (not a rude one) before they re-emerged for the seconds half.It's slightly awkward, see for yourself in the video below.

Real were then given a helping hand by prolific defender Sergio Ramos, who put Real back on level terms just over 10 minutes after the restart. Mertens then put through his own net before Alvaro Morata guaranteed victory on the night.

However, things aren't looking so good for Ronaldo in front of goal, as his failure to score means he has now gone six consecutive games in Europe without finding the back of the net.

This is his worst run in front of goal since he was at Manchester United, so he will no doubt be looking to rectify that in the next round.

He did lead from the front, though, and even while his performance belied his changing influence on the Madrid squad, he won't be happy with himself for not scoring.