Ronaldo suffered a minor knock to his right leg in the 3-1 La Liga win over Osasuna on Saturday and stayed in the gym as a precautionary measure.
"Real Madrid continued to prepare for the first leg of the Champions League round-of-16 tie against Napoli," a brief statement on the club's website reads.
"Cristiano Ronaldo and [Fabio] Coentrao trained inside the facilities.
"Ball work took centre stage in a session that saw [head coach Zinedine] Zidane call up Real Madrid Castilla player Enzo. Madrid completed the first session of the week performing various possession and pressing exercises, ball circulation and crossing and finishing."
Ronaldo's physical problems are not expected to keep him out Wednesday's encounter at the Santiago Bernabeu, with the Portuguese likely to resume regular training on Tuesday.
The 32-year-old has netted 21 goals in 26 appearances in all competitions for Madrid in 2016-17, but the attacker has scored just twice in the Champions League this term. He was crowned the competition's top scorer last term with 16 strikes in 12 outings.