After a summer of speculation that saw him linked with virtually every top club in Europe, the 18-year-old moved to Paris on an initial loan deal from Monaco ahead of a scheduled £166m permanent move next summer.
"Kylian is doing just fine, during the training sessions he is good and fit, but I don't know what happens in his head," said Rabiot, who has been with PSG for seven years.
"There have been a lot of changes since he arrived, both on the pitch and in the media, and he is so young. It could be difficult [for someone to cope with].
"But I think he is fine, he looks happy and if he is less good, it's only physically."
The Frenchman is on course to become the second most expensive player in footballing history when his permanent transfer goes through ahead of next season.