Ireland's Johnny Sexton converts a try during their Rugby World Cup opener against Scotland on Saturday. Photo: Rebecca Naden/Reuters

TOKYO – Ireland's Johnny Sexton is being monitored ahead of Saturday's clash with Japan and should return to full training on Thursday after receiving a blow to the thigh in his side's opening Rugby World Cup win, kicking coach Richie Murphy said.

Ireland's influential flyhalf relinquished kicking duties in the first half of Saturday's 27-3 win in Pool A over Scotland before departing midway through the second half in what Ireland said was a tactical change.

Murphy said Sexton was rehabilitating on Tuesday and that his understudy, Joey Carbery, was ready to play if needed after completing his recovery from an injury suffered in the build up to the tournament.

"He (Sexton) has done some work with the team but has been sort of monitored. We'll expect to train fully on Thursday and that wouldn't be unusual for him not to train two days after a Test match," Murphy told a news conference.

"He got a knock on the quad and when you kick, you kick with your quad. He just didn't feel comfortable to kick during the game so we have no problem passing over the duties."

Murphy added that flanker Peter O'Mahony and centre Bundee Aki  who were forced off with head injuries in the first half against the Scots  passed their final concussion tests, making both available for Ireland's second game.

Fellow centre Robbie Henshaw, who sustained a hamstring injury shortly after Ireland's arrival in Japan, also came through a session of running no problem, Murphy said.

Reuters