Joe Schmidt allayed injury fears over Johnny Sexton following Ireland's commanding Rugby World Cup win over Scotland but Bundee Aki and Peter O'Mahony are in a race against time to face hosts Japan.

Sexton, the reigning World Rugby Player of the Year, required treatment on his right thigh in the first half of the 27-3 victory in rainy Yokohama and headed to the bench just before the hour.

The star fly-half gave up kicking duties to Conor Murray to spark concerns of a muscle problem but Ireland coach Schmidt assured the change was purely a precaution.

"He was able to kick. That's us being super cautious," Schmidt told RTE Sport.

"Because we got such a good start we felt we could cover him up a little bit.

"We got him and Conor off reasonably early. I thought they were really big for us in the pivot area today with their kicking game and their game sense.

"I think they controlled things really well for us in that first half, got us through the first 15 minutes of the second half and then I felt we were probably far enough in front and close enough to that last quarter that we could bring them off and get them ready for a six-day turnaround, which is going to be tight with Japan having an eight-day turnaround."

There is less certainty over Aki and O'Mahony, who both failed head injury assessments (HIA) and are now in doubt for Saturday's second Pool A game.

Schimidt added: "They're both fine now, which is reassuring. But we'll have to wait. They have to go through the protocol of HIA 2 and HIA 3 to determine whether or not they've suffered a concussion.

"For us, six days to turn around is the minimum after a concussion.

"We'll see how they are symptomatically. But right here, right now there is a bit of reassurance over those two."

Injuries proved to be Ireland's only problems in their tournament opener as the world's top-ranked nation scored four unanswered tries to secure a comfortable bonus-point win, much to the satisfaction of captain Rory Best.

"Obviously with the [wet] conditions in the second half it can become a bit more of an arm-wrestle but we know how good of a team [Scotland] are and we're delighted with that win," Best said.

"We're under no illusions, that was a good start for us but we know and Joe and the rest of the management will show us tomorrow how much we need to improve going forward."