The Jeorge Bird's Arsenal youth website reports Ballard will be out of action until March.
The centre-back has been a mainstay in the Millwall team this season, making 19 appearances in the Championship and a further two in the Carabao Cup.
However, Northern Ireland international Ballard is now set for a spell on the sidelines and required surgery on his knee, as revealed by Millwall on their official website.
Ballard, who was previously released twice by Arsenal before bouncing back to earn a deal, firstly went on loan to Swindon Town, but that move was scuppered by injury.
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