Milan health coordinator Jean Pierre Meersseman announced the trip in an interview with the Web site Controcampo, saying Ronaldo would visit specialist Marc Martens.
The Brazil striker's knee problem ruled him out of Milan's first two games of the Serie A season - as well as the European Super Cup win over Sevilla last week.
Milan coach Carlo Ancelotti has said he expects Ronaldo back for Milan's next game against Siena on Sept. 15 - or when the squad opens defense of its Champions League title against Benfica three days later.
Ronaldo transferred to Milan from Real Madrid in January and was ineligible to play for the squad when it won the Champions League last season because he began the competition with the Spanish club.
Rescuers found the pilot of one of the two Su-34 fighters that had collided in midair in the Far East on January 18