The 28-year-old has played just once this season, against Empoli in September - a game that saw him withdrawn at half-time after suffering from dizziness.
Subsequent tests revealed that an accumulation of fluid, resulting in swelling on the brain, was the cause of Castan's discomfort.
Castan will now undergo treatment for the issue, but Garcia said he would leave it to the medical professionals to put a timeframe on his comeback.
"I'm not a doctor," he said. "There are professionals who will evaluate him. We need to leave him alone, but it's clear, he's a Roma player.
"We are all behind Leo - after his surgery in December he will return to being a Roma player.
"Now we must leave him alone. He’s a warrior; he will soon be back at 200 per cent."
Castan's problem is the latest in a long line of injuries for Roma this season, and Garcia has bemoaned his side's fitness fortunes.
"It's bad luck for sure," he added. "At the moment it's not something to worry about. With so many games to play, and some players playing too much, it increases the chances of injuries.
"Things are slowly getting back to normal, and that’s the important thing. The returning players will give us even more."
Roma currently sit second in Serie A and visit Atalanta on Saturday.
