Fit-again Ronaldo to start against Osasuna
The forward sustained the injury in Real's 0-0 draw at Real Mallorca on the opening day of the season and missed Portugal's European qualifiers at home to Cyprus and away to Norway.
"He'll play," Mourinho told a news conference on Friday after Ronaldo successfuly completed a training session at the club's facility on the outskirts of the Spanish capital.
"He's in good shape and because it's a difficult match he'll play from the start."
Mourinho will be without midfielders Kaka (knee) and Fernando Gago (knee) and defenders Raul Albiol (ankle) and Ezequiel Garay (knee), who are all injured.
Kaka, who has been plagued with injury and has failed to live up to expectations since joining Real from AC Milan for the start of the 2009-10 campaign, had surgery on his knee last month and is expected to be out for another three months.
The Brazil playmaker told Real's website his recovery was progressing well.
"I have no pain in the knee," he said. "It's a month now since the injury and everything is going well. I am doing many hours of physiotherapy and that's helping me a lot."