Norwich have revealed defender Timm Klose is set for a "lengthy" spell out with knee ligament damage, while midfielder Tom Trybull will need around six weeks to recover from an ankle problem.
Swiss centre-back Klose has missed the start of the new Premier League season because of hip trouble, but made a return to action in last week's Carabao Cup defeat at Crawley.
Klose, though, was forced off after just 27 minutes into his comeback after colliding with a Crawley player.
On Tuesday evening, the defender posted an update on social media, saying he had to "take a break for a few months", adding: "Time for me to fix the problem, recover and come back stronger than ever."
Norwich then confirmed on Wednesday scans have revealed "significant damage to the posterior cruciate ligament in Klose's right knee, with further specialist consultations ongoing." No timescale for recovery was mentioned.
Head coach Daniel Farke told canaries.co.uk: "It's a tough time for Timm and our thoughts are with him.
"The nature of the injury means it isn't a straightforward process, but he's in the care of the best possible people.
"We know the type of person Timm is. He'll work at his very best to come back from this setback and he will of course have our full support during this time."
Trybull, meanwhile, will not be available for approximately six weeks due to an ankle ligament injury which was sustained during Saturday's 2-0 defeat at West Ham.
Centre-back Christoph Zimmerman made his first Premier League appearance at the London Stadium after a recent hip problem, but picked up a foot ligament injury against the Hammers which Norwich's medical team are continuing to assess.
Scotland defender Grant Hanley is also being treated for a groin problem over the international break.
Farke added: "Yes, of course it is a blow for us and the players, but these are some of the challenges we have to face right now."
Norwich won the Sky Bet Championship last season to secure promotion back to the top flight.
However, Farke's side have lost three of their opening four league games and next face champions Manchester City at Carrow Road on September 14.