"It's very, very disappointing," said the Canaries manager.
"He is a big loss for us but, unfortunately, that's football.
"It was just one of those accidental football collisions in training that happens.
"Just after it happened it was very, very sore and once it started to swell up it was a case of getting him to hospital for an X-ray and that's when it was revealed that he'd got the fracture.
"Damien will have an operation on Saturday and then, and only then, will we have a more accurate timescale as to how long he might be out for.
"It could be one of those that, if everything settles down quickly, with a mask he could be back within 14 days.
"But if the surgeon gets inside and it's more complicated then it could be six weeks, two months.
"Obviously we're all disappointed, but particularly for the lad himself. Damien's playing at the peak of his performance and his goals have been very important for ourselves."