Powell suffered the injury last week in a training match and the 22-year-old has missed the Dons' first two games of the season against Shrewsbury and Northampton.
"It's catastrophic what's happened to him," Dons boss Karl Robinson said. "It was a big blow to me personally to find out.
"It looks like it's going to be 10 weeks. It was innocuous, the last kick of the game."