After one game back from concussion, Charlie Ngatai has been ruled out of the Chiefs' match against the Crusaders in Fiji.

Just as it was looking all rosy for Charlie Ngatai, there are more concerns.

The Chiefs midfielder has again been sidelined by concussion symptoms, after just one Super Rugby game back, following a year out.

Having returned to start in the 46-17 win over the Reds in New Plymouth a fortnight back, Ngatai is unavailable for the match against the Crusaders in Fiji on Friday night, after developing a headache post-training on Monday afternoon, which kept him from travelling with the team on Tuesday.

Assistant coach Andrew Strawbridge said it wasn't known whether this latest ailment was related to Ngatai's previous concussion battle, and the coaching team had checked footage and hadn't been able to identify anything obvious which may have brought on the symptoms.

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"Charlie developed a headache after training the other day, and we're not sure why, but we're taking a precautionary approach, so that's the reason he's not playing," Strawbridge said.

"We want to look after him."

It's either just a shocking coincidence for Ngatai to pick up the headache, or perhaps a very unfortunate and worrying sign of more trouble. It's something which time will hopefully tell, and heal.

The 26-year-old has endured a massive ordeal already. All after his initial knock, against the Highlanders in Hamilton on May 7 last year, was seemingly so innocuous - even the TV cameras didn't pick it up. That was just as he was on the verge of adding to his one All Blacks cap.

Returning via club footy, Ngatai then showed signs of his explosive best in his 55 minutes in New Plymouth - that 50-metre surge such a sight, then scoring a try - so fitting on his provincial home turf, 364 days on from the day he was derailed.

In a weird irony, while Ngatai is sidelined, co-captain Aaron Cruden has recovered from his own concussion symptoms and been named to start. His return means despite a stunning display in his late callup against the Reds prior to last weekend's bye, Stephen Donald drops to the bench for Suva.

Strawbridge said rather than use Donald at second five-eighth, they picked players who they felt would do the best job in what he envisaged would be increased physical roles for the midfield this time out.

Anton Lienert-Brown moves in one and Sam McNicol comes in at centre, while there's also a change on the right wing, with Toni Pulu sidelined with a hamstring strain and Tim Nanai-Williams returning, having overcome the shoulder injury he sustained in round three against the Hurricanes.

"Tim's one of our best outside backs - he has been for a long, long time... he gives us speed and power and workrate and elusiveness and aggression in that position, and he's been training really well for a while now," Strawbridge said.

After having a run off the bench after nearly nine months out with a knee injury, hooker Nathan Harris has been promoted to start in a pack which sees Mitchell Brown keep Liam Messam on the bench and which will be severely tested by the rampaging, 11-0 Crusaders.

Meanwhile, No 8 Michael Leitch has confirmed this season will be his last with the Chiefs. As reported by Stuff last month, Japan international Leitch was keen on linking up with the Sunwolves in either of the next two seasons, ahead of the 2019 World Cup, and he said with the Chiefs wanting to look to the future and sign their players soon, he had to make a decision.

New coach Colin Cooper has recruited Canadian back-rower Tyler Ardron, and teams can only carry two overseas players (plus one Pacific Island international), but only one per position, ie: loose forward.

Chiefs: Damian McKenzie, Tim Nanai-Williams, Sam McNicol, Anton Lienert-Brown, James Lowe, Aaron Cruden (co-c), Tawera Kerr-Barlow, Michael Leitch, Sam Cane (co-c), Mitchell Brown, Brodie Retallick, Dominic Bird, Nepo Laulala, Nathan Harris, Kane Hames. Reserves: Hika Elliot, Siegfried Fisi'ihoi, Atu Moli, Taleni Seu, Liam Messam, Jonathan Taumateine, Stephen Donald, Shaun Stevenson.