Leicester's director of rugby Richard Cockerill confirmed that Castrogiovanni fractured his rib during the first half of the Six Nations fixture which England won 19-15 in Rome's Olympic Stadium.
"The initial diagnosis is four to six weeks, and that's bad luck for Castro," Cockerill told the Leicester Mercury.
"He was really up for the England game and carried the ball a lot.
"But then he fell on the ball, got a bang in the ribs and he has cracked one."
The injury means Castrogiovanni could miss the rest of Italy's Six Nations campaign but he could return to action for Leicester in the LV= Cup and Aviva Premiership competitions before the end of next month.
Meanwhile, Italy coach Jacques Brunel has replaced Castrogiovanni with Aironi prop Fabio Staibano for the Six Nations clash with Ireland in two weeks time.
Staibano has represented Italy in nine Tests but hasn't played since coming off the bench in a 27-6 loss against the All Blacks in Christchurch back in 2009.
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