Lucas played both of Madrid’s matches in Japan but was replaced nine minutes from the end of normal time in the final, which saw Zinedine Zidane’s men beat Kashima Antlers 4-2 after two additional periods.
And the winger will now undergo treatment on his grade two calf injury ahead of Madrid’s return to competitive action against Sevilla in the Copa del Rey on January 4.
A club statement read: “Following the tests carried out on Lucas Vazquez in the Sanitas la Moraleja University Hospital, the player has been diagnosed with a grade two injury to his right soleus muscle, an injury he picked up at the Club World Cup.
“His recovery will continue to be assessed.”
In addition Mateo Kovacic has been diagnosed with a sprained ankle and his recovery will also be monitored.
Lucas has scored one goal in 23 appearances for Madrid this season, while Kovacic has made 18 outings with the same goal return.