Legzdins sustained the injury in training earlier this month and as a result missed the recent matches against Millwall, Bolton and Reading.
After undergoing a series of tests last week it was confirmed that the 26-year-old had ruptured his ligaments and he underwent surgery, with the rehabilitation process ruling him out for around eight weeks.
"It's obviously a bit of a blow for Adam and he has undergone surgery now," Derby manager Nigel Clough told the club's official website.
"He has been a good understudy to Lee Grant this season and pushed him all the way since the beginning of pre-season, but Saul Deeney will now step up and do exactly the same during the next few weeks."
Legzdins made 33 appearances last season after establishing himself as the Rams' first-choice keeper ahead of Frank Fielding.