The 37-year-old left the Boston Celtics over the summer, and although he has previously revealed that he does not speak to former teammate Ray Allen, the same does not apply to Rondo.
"It's always good to see your little brother, man. I worry about his health. I know he wants to play. I know he wants to help that team. I try to keep an eye out for the young boys [in Boston], just to see how they're doing," Garnett said, the New York Post reports.
Rondo, who recently returned from an ACL injury which had sidelined him since last January, will face his former teammates in Boston on Sunday.