The 26-year-old sustained the injury in training earlier this month and underwent surgery last week.
It is expected that the rehabilitation process will keep Legzdins out for around two months.
“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.”
