The Brazilian leader met the 81-year-old Castro in Havana, the Cuban capital, on Tuesday, and the two leaders discussed "all topics."
"I think Fidel will soon assume his political role in Cuba, his historic role in Cuba and the globalized world," Lula told reporters as he prepared to fly home from Havana's international airport after his 24-hour visit to the Caribbean island.
Castro underwent gastrointestinal surgery in July 2006, subsequently handing over 'temporary' power to his brother, Raul Castro, 76.
The leader of the 1959 Cuban Revolution has not been seen in public since, although he has made a number of television appearances. He is also due to run in parliamentary elections in Cuba on Sunday, a prerequisite for holding onto power.
During his visit to Cuba, under a U.S. trade embargo for almost half a decade, Silva signed a deal for Brazil's state energy company, Petrobras, to drill for oil in Cuban waters, which could potentially hold large quantities of crude.
Spanish, Canadian, Indian and Malaysian companies had previously agreed on deals to explore the area.
Silva also signed an aid package for Cuba said to be worth up to $1 billion.
