Newcastle United can confirm that Siem de Jong has suffered a pneumothorax (collapsed lung), similar to a previous complaint he suffered from while with Ajax in 2013.
De Jong has undergone surgery for the condition today (Tuesday) and is expected to be out for eight weeks.
The Dutch international said: "After four months of hard work getting back to full fitness it is such bad luck to suffer another setback.
"We are all hopeful that the surgery I have had will ensure I don't have any repeat of the condition in the future.
"It's very sad for me personally, especially since I was almost back in contention and feeling great. I'm sorry for my teammates, the Club and our fans that I'm not able to help the team for a little while longer.
"I am determined to come back stronger as soon as possible. Thank you to everyone for your support."