Ryan says:
"I'm not exactly sure [what the injury is], but it's nothing serious. There was no ligament damage or anything else. It's probably more of an inconvenience than anything else. Anytime you deal with a knee though, it's pretty scary. He should be fine."

http://profootballtalk.nbcsports.com/2009/08/09/nick-mangold-suffers-knee-injury/