Ronaldo scored the opener in Real Madrid's 3-1 victory over Osasuna. Getty

Cristiano Ronaldo has set alarms bells ringing at Real Madrid ahead of the visit of Napoli on Wednesday (15 February) after being missing full-squad training on Monday morning. Los Blancos have failed to disclose the reason of his absence but reports in Spain suggest that it could be a precautionary measure, with the current Ballon d'Or still in line to help his side in the first leg of the Champions League last 16.

Ronaldo, 32, has been under some scrutiny at the Santiago Bernabeu in recent times after his form suffered a dip in the first part of the 2016-17 season.

However, the Portugal international looked set to arrive for the game against Napoli in good shape following an impressive performance during Saturday's 3-1 victory over Osasuna. The former Manchester United star played the full 90 minutes and scored Real Madrid's opener to help his side remain at the top of La Liga, one point ahead of Barcelona with two games in hand.

The good news for Real Madrid continued on Sunday when the club revealed that Gareth Bale had returned to full training for the first time since suffering a serious ankle injury during Champions League victory over Sporting CP on 22 November.

But that news has now been overshadowed after the club revealed that both Ronaldo and Fabio Coentrao failed to train with the rest of Zinedine Zidane's charges on Monday morning, as Real Madrid continued the preparations for the visit of Napoli.

"Los Blancos completed the first session of the week, performing various possession and pressing exercises, ball circulation and crossing and finishing. Cristiano Ronaldo and Coentrão trained inside the facilities," the club confirmed.

Both Marca and AS are suggesting Ronaldo's absence from the session was just precautionary and he should be ready to play and start against the Serie A side on Wednesday night.

Meanwhile, Bale trained again with the other available players, but it is still uncertain whether the former Tottenham Hotspur star will be 100% fit to host Napoli.