The 29-year-old Penrith Panthers forward was hurt in the second half of the final Group C match, which Italy needed to win to go through to the quarter-finals.
Ciraldo was sent crashing to the ground in a thunderous tackle by Tonga centre Konrad Hurrell and had to be helped from the pitch.
He returned after the match with the team to their Leeds hotel, but after complaining of back and stomach problems, he was taken to hospital by ambulance and admitted.
The Italian management say he will stay behind when the rest of the squad return home on Monday night and is likely to need surgery.
"Italy team manager Reno Santaguida will remain in Leeds to ensure Ciraldo is assisted in his recovery," said a spokesman. "Penrith coach and New Zealand assistant coach Ivan Cleary is concerned about Ciraldo's health and will visit him later today."
Ciraldo, who has won six caps for Italy, is due to take up a position in coaching and development at Penrith and his parents were at The Shay to see their son's final game. They then visited him in hospital late on Sunday night.
- Sports & Recreation
- Penrith Panthers