The Roma man limped off in the closing stages of the first half after what looked like an innocent clash with a Dutch opponent.
Zaniolo only returned from a tear of his anterior cruciate ligament back in July, but there are fears he may have suffered the same injury to his other knee.
“It’s a significant sprain of the knee,” Ferretti told RAI Sport.
“It’s tough to compare the stability with the other knee because it has already undergone a surgery. It’s a significant sprain.
“He is coming back from a long rehabilitation process for the other knee, so he is obviously worried, as are we.”