Johnny Sexton has branded Ireland's stunning loss to Japan a "blessing in disguise" because it happened in the World Cup's pool stages.

Fly-half Sexton has declared himself fit after a thigh issue to face Russia in Kobe on Thursday, where Ireland will battle to set their Pool A campaign back on track.

Chris Farrell has been ruled out of facing the Russians due to concussion, while Robbie Henshaw remains a long-shot to feature as he continues to battle a hamstring problem.

Ireland suffered one of the all-time great World Cup shocks in Saturday's 19-12 loss to hosts Japan in Shizuoka – but Sexton insisted Joe Schmidt's men must be thankful their hefty defeat did not occur in the knockout stages.

Sexton was at pains to explain how Ireland swept through the pool stages in 2011 and 2015 only to be dumped out both times in the quarter-finals – by Wales in New Zealand eight years ago and by Argentina four years back.

"The only blessing in disguise is that in the last two World Cups I've been involved in, we've not cruised through the group, but we've had everything go our way in the pool stages," said Sexton.

"And then we've had the day we had yesterday in the quarter-final. And we'd be going home today.

"So the great thing now is that we've got the rest of the pool to get things together.

"We know to a certain extent things are in our hands if we can win the last two games and score a number of tries that we can definitely qualify for the quarter-final.

"First or second place is out of our control, so we won't worry about that.

"But it's up to us now to put in two big performances and hopefully get ourselves in to a quarter-final. And we know that if we can get there we've got a chance.

"We've responded really well in terms of the other recent setbacks.

"At the start of the Six Nations we turned things around, had a brilliant performance against France.

"Things went badly against England (in August's record 57-15 loss at Twickenham) but we beat Wales back to back and started with a bang against Scotland.

"A five-game winning run now would be great, that would do nicely."

Sexton missed Ireland's stunning loss to Japan due to a thigh knock suffered in the 27-3 bonus-point thrashing of Scotland that opened his side's World Cup account.

The 34-year-old insisted however that he is now ready to get back to action, and is itching to help the Irish squad move past their damaging defeat by the Brave Blossoms.

Ireland flew into their second Pool A clash with Japan intent on taking a giant leap towards quarter-final qualification, only to be struck down by boss Jamie Joseph's inspired Brave Blossoms.

While Ireland ought to ease past Russia without issue, Sexton is desperate to get back into action as soon as possible.

"One thing about this team is that we back the squad, the 31 guys, we said that when we picked the squad, everyone here is ready to play," said Sexton.

"I picked up that strain against Scotland, and I wasn't going to train fully Monday, Tuesday.

"The preparation time was so short the guys made the decision for me to sit this one out.

"You want to play every game but hopefully I can pay back the squad this week and next week too."

Centre Farrell's absence against Russia could push Bundee Aki back into service, with the Connacht midfielder expected to be available after his own head injury issues.

"Chris Farrell has a confirmed concussion, so he won't recover in time for Thursday," said Ireland forwards coach Simon Easterby.

"Rob Kearney passed his HIA two so he's in that process.

"Tadhg Furlong came back onto the pitch, so he's fine as well. Those were the three obvious ones from an HIA perspective.

"Robbie (Henshaw) ran really well yesterday but we obviously have to take a view on his progress when we get back to training; he's doing really but we'll have to see."

Ireland have also drafted Ulster's Jordi Murphy into their World Cup ranks, to replace Jack Conan, who will miss the rest of the tournament with a foot fracture.

"Jack's a quality rugby player and can break the game up, he's a bit of a freak with the ball in hand so he's a real loss," said Easterby.

"But we are fortunate that Jordi Murphy is joining us, he gives us great coverage especially as he can play anywhere across the back-row."