The 33-year-old - who has over 100 caps to his name since making his debut in 2002 - was bitten on the nose by a friend's dog at the weekend and had 14 stitches inserted in the wound.
It was described as a "domestic incident" by Italy team bosses, who on Tuesday confirmed the Toulon prop would miss the trip to Murrayfield as he continues a course of antibiotics.
"Castro is not training so as to avoid any risk of infection, and is still following a course of antibiotics," said team manager Luigi Troiani. "He will not be available for the trip to Scotland, but we'll be counting on him returning as soon as possible."
Lock Marco Bortolami has also been ruled out of the Murrayfield match as he continues his recovery from a muscle strain in his right leg.
"We decided to keep Marco with us at training camp until our departure for Scotland despite the fact he won't be playing in the game. He will continue to undergo treatment with his club, Zebre," added Troiani.
Benetton Treviso lock Marco Fuser, who has been suffering from a right shoulder injury, will be available for selection having taken part in recent training sessions, he added.
Italy still have doubts on wing Leonardo Sarto and centre Luca Morisi. Both have been forced to train apart from the rest of the squad due to hamstring and thigh injuries respectively. Both are expected to be reassessed to see if they can be included in a squad training session on Wednesday, said Troiani.
Italy, who finished with the tournament's wooden spoon for their last place finish last season, started this year's tournament with high hopes but have lost to both Ireland (3-26) and England (47-17).