The 29-year-old missed Wednesday's Copa del Rey final victory over Barcelona with a knee injury but has now returned to training.
The 29-year-old missed Wednesday's Copa del Rey final win over Barcelona with a knee injury and looked set to be absent for the first-leg of his side's last four European tie Wednesday.
However, he could now be available for the clash with Pep Guardiola's side after coming through the first part of Sunday's training session.
Madrid boss Carlo Ancelotti will also be encouraged by the participation of Marcelo, who emerged unscathed from the practice after been sidelined with a hamstring problem.
Copa del Rey hero Gareth Bale was also present following his match-winning heroics midweek, while defender Sergio Ramos worked alone inside the club's training facility.
The news that Ronaldo could be back for the key clash will not only come as a relief to Madrid, with Bayern CEO Karl-Heinz Rummenigge saying that he was eager to see the Portuguese star face his side.
"When Real Madrid play Bayern Munich, then the whole world will be watching. I always want the best players to be on the pitch," he told TZ.