Ronaldo has been named in Real Madrid's squad for the first time this season.
In the squad for Tuesday's European Champions League Group E match with Dynamo Kiev, Ronaldo is available for his first appearance since Brazil's 1-0 loss to France in the World Cup quarterfinals in July.
Ronaldo has spent the last two months recovering from surgery to remove calcifications from his left knee and hadn't been expected to return until next Sunday when Madrid hosts crosstown rival Atletico in the Spanish league.
The striker was called up as cover for striker Ruud van Nistelrooy, who was given leave to attend the birth of his child.
Van Nistelrooy became father to a daughter overnight and his return to training Tuesday suggested he will be included in the starting lineup, news reports said.
Madrid coach Fabio Capello now has to decide which player to omit from his 19-man squad, with leading sports daily Marca saying on its Web site it will be either Ronaldo or reserve forward Javi Garcia.
Ronaldo, who turned 30 last Friday, has yet to play under Capello. The Brazilian attracted interest from AC Milan in the offseason but the clubs couldn't agree to a fee, reports AP.
Madrid badly needs to beat Kiev at Santiago Bernabeu Stadium, having lost its opening Champions League match against Lyon 2-0 two weeks ago. The Spanish team is third in Group E.