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British and Irish Lions captain Sam Warburton is a doubt for Saturday's tour opener against the Barbarians.

Assistant coach Rob Howley said the Wales flanker picked up a knock on his knee and will continue to be assessed after missing Wednesday's training session.

Prop Gethin Jenkins has already been ruled out of the opening warm-up match in Hong Kong with a calf injury.

"We just want to give Sam a bit more time before Saturday," said Howley.

"He took a bump in training last week and it is just a bit puffy so we will assess that over the next 24 hours."

Head coach Warren Gatland is due to confirm his starting line-up on Thursday at 05:30 BST.

If Warburton is deemed unfit, Ireland lock Paul O'Connell - Lions captain in 2009 - is likely to lead out a side containing at least eight Welshmen.

England duo Owen Farrell and Mako Vunipola, and Scotland lock Richie Gray are also set to start at the Hong Kong Stadium.

Gray, who hasn't played since injuring a hamstring against Wales in the Six Nations on 9 March, Wales wing Alex Cuthbert - out of action since facing England on 16 March - and flanker Dan Lydiate, who has only played a handful of club games after six months on the sidelines, are among those in most need of game time.

"As Lions, you want to get that first game under your belt and it's a great opportunity for the guys starting on Saturday to put a marker down," added Howley, who reiterated that every player will start at least one of the first three games.

"It will be a good hit-out against a very strong Barbarians side and you don't get that many opportunities before the first Test so it is important you hit the ground running."

The six Leicester and six Leinster players, plus Ulster's Tommy Bowe, who were involved in domestic finals last weekend trained with the squad for the first time on Wednesday, but none is likely to start Saturday's match.

However, Sean O'Brien, who missed Leinster's Pro 12 victory over Ulster with a knee problem, could be involved in the match-day squad.

The main challenge for those involved on Saturday will be the extreme heat and humidity, with temperatures around 30C and humidity up to 90%.

The players used a phalanx of giant portable fans spraying out water to keep cool during training on Wednesday.

"The heat is going to take its toll and the stress they will be under will be huge, but that is something that will benefit us when we get to Australia," Howley said.

"The players might not feel like that about it this weekend but it will put their decision-making under pressure as well, which is good for us."