Double Olympic champion Andrew Triggs Hodge will not row again this year after a recurrence of glandular fever.
However, he has not trained in 2015 and has conceded he will not be ready for September's World Championships in Aiguebelette, France.
"It would take a miracle to get into form and shape," he told BBC Sport.
"Initially, I was thinking, 'right, we'll see how I am in a month', then it became two months and now it's become obvious I'll miss the Worlds.
"I'm having fortnightly blood tests and just waiting to get better so I can return to training."
Glandular fever can cause extreme tiredness and, while Triggs Hodge is still targeting a third consecutive Olympic gold in Rio, he concedes the timing of his absence has cast some doubt on those ambitions.
"I love training and making boats go fast but now there is a question about whether I can commit to an Olympic programme," said Triggs Hodge, who will be part of BBC Sport's commentary team for the Boat Race on April 11.
"You have to prioritise yourself and be selfish to get a gold medal but at 35 with a wife and kid other things come into play.
"But right now I just want to get back to training and see what happens."