The 25-year-old fractured his football wil playing for Middlesbrough against Villa in May.
Downing joined Villa in a £12million deal earlier this summer.
“I’ve had to have it pinned and that should come out in September. Then it’s a case of building it up,” he said.
“I’m sure I will play before Christmas. The manager [Martin O’Neill] wants me 100% right and is prepared to give me extra time to do it,” Downing added.
“The injury is a bit of a strange one. It’s not a break but there’s a fracture there.
“The target is early December. If I’m ready before then that’s a bonus. If it’s after it just needs time to heal.”
