The defender, who joined Manchester United on deadline day, was forced off in the 1-1 draw against Leicester City on Sunday.
Speaking to MUTV on Tuesday morning, ahead of the PSG match, Skinner said: "It's an ACL. There are different levels of ACLs.
"She goes in today for an operation.
"I would always put 12 months on this sort of injury. She could come back sooner, but I would rather work back.
"That's the sensible way to prepare, working backwards.