Six Nations - Castrogiovanni to miss rest of Six Nations
The long-haired symbol of Italian rugby is famous for his toughness but he was in severe discomfort when he came off midway through the match.
"It could be a broken rib and if that is the case it would be difficult to have him back during the rest of the tournament," coach Jacques Brunel said with later media reports saying the fracture was confirmed.
The news about Castrogiovanni made the day worse for captain Sergio Parisse whose modest side were on the verge of a shock triumph to match last year's win over France when they led 15-6 early in the second half.
"I'm really disappointed as we could and should have beaten England, also for this marvellous crowd which supported us from the start despite the snow and cold," he said after Italy's first Six Nations game at Rome's Stadio Olimpico.
"It is a chance lost. They weren't at their best and the precarious conditions could have been to our advantage. It's a shame."