The 28-year-old, who agreed to join Juve in January, started just four league games for Athletic Bilbao last season due to a rift with the club.
However, the Spanish striker told reporters at his official unveiling on Tuesday that there should be no concern about his fitness.
"Physically I feel very well. I have worked very hard during my holidays and now I am ready to work even harder in the future," he said.
"Over the last two years I have still done a lot of physical training even when I have played much less. I know Italian football is very difficult, but I am sure I will be able to do well."
Llorente will form a new-look attacking partnership with Carlos Tevez next season.