Chris Boyd has revealed Hurricanes have no idea how captain Dane Coles has come to be suffering from symptoms of concussion.
There are concerns for New Zealand hooker Coles ahead of the start of the British and Irish Lions series next month after he complained of headaches.
Hurricanes head coach Boyd said in April he would be "pretty surprised" if the inspirational skipper did not make his Super Rugby comeback against Stormers on Friday following an absence of over a month due to knee and calf injuries.
But the 30-year-old's return has been delayed for reasons that are unknown to the Wellington-based franchise.
Boyd said: "Colesy's situation is complicated by the fact that... subsequent to his knee injury, he not only developed a calf strain but also some headaches.
"He is now under a return to play protocol for concussion. We genuinely don't know where he got those symptoms from.
"We've had a look at the Highlanders game, which was the last game he played. He had no symptoms post that, so I'm not sure. The source of that is completely unknown to us."
He added: "At the end of the day, he has started to return to train and developed some headaches.
"It's not normal, but if you look at it in context, because of his knee injury, the first part of his rehab was no training at all so, if there were symptoms post the game that would come out under exercise, they were not apparent."