Ridgers was carried off during Scotland Under-21s' 1-0 friendly defeat by Belgium on Tuesday after a collision with Thomas Meunier.
Scans have shown the 21-year-old has torn his posterior cruciate ligament and suffered further ligament damage.
Hearts say Ridgers, who began the season as second choice to Jamie MacDonald at Tynecastle, will now visit a specialist to assess whether surgery is required.
Manager John McGlynn told Hearts News: "I am gutted for the boy. It's the start of the season, he's been doing well and now he's looking at a minimum of 12 weeks out.
"It is a blow for him and the club and we all wish him a speedy recovery.
"Hearts will give him all the help and support we can to get him back fit as quickly as possible."
Scotland Under-19 goalkeeper Jack Hamilton will step up to act as cover for MacDonald.
Ridgers will miss the under-21s' final European qualifiers against Luxembourg and Austria.
With St Mirren's Grant Adam out with a fractured leg, Tottenham keeper Jordan Archer looks set to continue for Billy Stark's team after making his debut as a replacement for Ridgers on Tuesday.
