Leeds winger Ben Jones-Bishop has been ruled out of action for up to six months while he undergoes treatment on a blood clot affecting his lungs.
The Super League champions announced that doctors have advised Jones-Bishop to rest for three months, avoiding all contact training.
Jones-Bishop, 24, said on Leeds' website: "It is obviously disappointing to miss the start of the season but I am looking forward now and doing everything I can to get back as soon as I can once I am given the all-clear."
He added: "I am only able to do very light training at the minute and I will have another scan in a couple of weeks to see if I can increase that but I know I have to avoid contact for three months at least.
"It is a relief to get it diagnosed and get treatment and to know that it is not a life-threatening condition. Generally I am in good health but as a sportsman you need to resolve these issues as soon as possible to make sure that you can perform to your maximum."
Leeds medical chief Dr Jon Greenwell expects Jones-Bishop to make a full recovery from the problem.
Dr Greenwell said: "He has had a pulmonary embolism which has led to him needing treatment for three months and a consequence of that treatment is that he cannot play rugby until after the treatment is completed and he has been given the all-clear by his specialist.
"Ben is still in light training but under advice from the specialist he is unable to play for three months. We are confident the treatment will resolve the problem."
A pulmonary embolism is a blood clot in the artery that takes blood from the heart to the lungs and Leeds coach Brian McDermott is resigned to being without the England Knights player for as long as proves necessary.
McDermott said: "We have no major long-term concerns that this will impact on Ben's career and he will receive the best possible care over the next three months to make sure that the issue is resolved and he can resume playing."