According to IGN:
While it might seem strange to pit Shang-Chi against a villain most
commonly associated with Iron Man, this choice backs up a popular
theory that the Mandarin will be depicted as Shang-Chi's father in the
The problems here are two-fold. One, Marvel no longer has the rights
to the Fu Manchu character. Not only has this made reprinting the
early Master of Kung Fu comics difficult, but it's also prevented them from
referencing Shang-Chi's parentage in subsequent comics. Two, Fu Manchu
is now widely criticized as a racist caricature and an example of the
"Yellow Peril" storytelling trope. Even if Marvel could bring Fu
Manchu into the MCU, it's doubtful they'd actually want to use a
villain with such a problematic history.
So Shang-Chi and Mandarin are not related at all in the comics, but rather Mandarin will be replacing a character that Marvel cannot (and probably should not) use.
Please note that they are referring to the "real" Mandarin (revealed in the Marvel one-shot All Hail the King), rather than the "fake" played by Ben Kingsley.