The future of Chiefs Super Rugby midfielder Charlie Ngatai remains up in the air as he continues to battle the symptoms of a serious concussion.

The one-Test All Black suffered a head knock against the Highlanders in May and has not played a game since then, missing the remainder of the Super Rugby season and New Zealand's national provincial championship.

Prolonged absence: Charlie Ngatai. Credit:Getty Images

Chiefs coach Dave Rennie told reporters that the 26-year-old was not back in full training and could miss the entire competition.

"It's been an incredible slow process and he's had a few setbacks over the last period of time," Rennie said.