Beckham has suffered a ruptured Achilles tendon after breaking down in yesterday's victory over Chievo Verona.
"He will probably be out for five or six months," lamented Galliani. "I saw him in pain. In the changing room I hugged him and I told him that if he wants, he can return to us next year. These incidents tarnish the joy of the win.
"Beckham came to AC Milan so to be called by (England coach Fabio) Capello, unfortunately, this is football .. but David has a great strength of mind."
Beckham, on-loan from LA Galaxy, will leave Milan today for Finland where he will be operated on by surgeon Sakari Orava.
Galliani added: "The doctor? He's a luminary in his field."