De Rossi suffered a knock to his right thigh during the last-16 tie with Spain on Monday and was replaced by Thiago Motta nine minutes into the second half in Paris.
However, while the Roma midfielder did not take part in Italy's training sessions on Tuesday and Wednesday, Castellacci refuses to rule out the player just yet.
"We are doing everything to get him back but time is running out," he said.
"We don't want to make premature conclusions even if we know how close the match is."
There is better news for Italy coach Antonio Conte over the fitness of Antonio Candreva, however.
The Lazio winger has missed his country's last two games with a groin injury but is making good progress in his recovery, according to Castellacci.
"The situation with Candreva is better but the muscle hasn't fully healed. There haven't been further setbacks," he added.