The Azzurri's medical department remains optimistic about the defender's recovery as the Juventus man may take part in the team's final group match against Ireland
The 31-year-old's involvement in the tournament had been in serious doubt after he picked up a calf strain which was feared that would take up to 20 days to heal.
As a result, Davide Astori was asked to cut short his holiday in Miami and join up with the rest of the squad on stand-by, but the Cagliari defender has now been allowed to return home.
"After some medical tests, we have noticed a considerable improvement with regards to the injury," Castellacci told reporters.
"Because of this, coach Cesare Prandelli has decided to keep him on the team, in the hope that he will at least be able to play in the third group match [against Republic of Ireland].
Prior to the match against Ireland on June 18, Italy will tackle Spain on June 10 and Croatia on June 14.