The England international was forced off the pitch in the closing stages at Goodison Park on Saturday shortly before his team's last-gasp equaliser in the 1-1 draw with Everton.
A post on Norwich's official Twitter account said: "City can confirm 'keeper @Johnruddy86 has a thigh injury which is being assessed, but he could be out for approximately 3 months."
Mark Bunn was Ruddy's replacement having already featured twice for the Canaries this season in their Capital One Cup wins over Doncaster Rovers and Tottenham.
Ruddy tweeted: "Just to update everyone on my injury, unfortunately I have torn my thigh will be having surgery on saturday and out for up to 3 months, :("
It is a huge blow for the 26-year-old who has shone for Norwich in their rise to the top flight and has become a regular in Roy Hodgson's England squad.
He arrived at Carrow Road from Motherwell in July 2010 having been on loan with a number of clubs, including Crewe, Stockport, Bristol City, Wrexham and Walsall.
He spent five years at the start of his professional career at Everton with the Toffees paying Cambridge United £250,000.