The joint La Liga top scorer missed Real's last two matches with a leg muscle strain but trained at the same pace as his team-mates ahead of the last-16 second leg.
Real are firm favourites to progress having won all their home games this season even though Lyon, who have not lost to the nine-times European champions in seven meetings and knocked them out at the same stage last term, fought back to draw 1-1 in the first leg.
Real coach Jose Mourinho would not say whether he will include Portuguese compatriot Ronaldo, who became the world's most expensive player when he joined Real from Manchester United in 2009, in his starting line-up for the game.
"I need to consult the medical department for a final opinion and to hear how the player himself feels, his final feelings in training," Mourinho said at a pre-training news conference at Real's vast facility outside Madrid.
"I am not going to wait until tomorrow," the former Inter Milan, Chelsea and Porto boss added.
"I don't like deciding the day of the match. I like to have everything clear in my head the day before. If he is fine he'll obviously play but we have to wait a bit longer to see."
Real hired Mourinho at the end of last season after he led Inter to an unprecedented treble of Italian league and cup and Champions League.
A 10th European crown has remained out of reach since their last title in 2002 and they have failed to progress to the quarter-finals of the continent's elite competition in the past six seasons.
They limped out of last year's edition when Lyon snatched a 1-1 draw at the Bernabeu to progress 2-1 on aggregate.
Mourinho said Real should keep a sense of perspective about winning their 10th title.
"You have to remain calm," he said. "We have to fight hard to make it this year, of course, but if not then next.
"It's not a serious problem. A serious problem is what is happening in Japan, not getting to the Champions League final."
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