The Portuguese star has now failed to complete his last three training sessions and left the field again on Tuesday after an incident involving defender Dani Carvajal.
The 31-year-old completed only 45 minutes of Real's final game of the season, a 2-0 win over Deportivo La Coruna 10 days ago in which he scored both goals.
The three-time Ballon d'Or winner has been beset by injury problems in recent weeks after limping off during the 3-0 win over Villarreal in April.
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But Ronaldo claims the problems is not major, calling it just a "scare", and believes he will be fit to take to the field against Atletico Madrid on Saturday night.
"I had a little scare... but in a couple of days I will be fine.
"I've had a little problem, but having been the most used player this season that is normal.
"I think it's the season where I've played the most minutes, and taking out this problem from training I am at my best."
Ronaldo tried to stretch out his left thigh following a clash with Carvajal, but left the training session early following a chat with physios and medical staff.

Ronaldo is a doubt for the Champions League final

Image credit: Reuters

OUR VIEW

Understandably the loss of Cristiano Ronaldo would be a huge blow for Zinedine Zidane and Real Madrid. Yet should he make it's quite clear the Portugal international is nowhere near full-fitness, which does not bode well for his Euro 2016 prospects either.
In the absence of Ronaldo, how do the 2014 champions line-up against their city rivals? Lucas Vasquez started in Ronaldo's place when he missed the first-leg semi-final draw with Manchester City and would appear to be front-runner again.
James Rodriguez and Isco surely come into the thinking, though Zidane has not given either much first-team opportunity so far in his reign.
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