Makuhari - Ireland's World Cup preparations have suffered a blow with centre Robbie Henshaw picking up a hamstring injury likely to keep him out of their opening match against Scotland.

Head coach Joe Schmidt told reporters on Sunday that the 26-year-old Leinster centre would have a scan later in the day.

"We'd be hopeful it's not too bad. Is it likely to rule him out of this week? Yeah, you'd have to say it is likely," admitted Schmidt ahead of Ireland's clash with Scotland next Sunday in Yokohama.

It's a case of deja vu for Henshaw, who also missed the first two games of the 2015 World Cup through injury.

"It could be a cramp. We've travelled a lot. We've tried to be fully rehydrated, fully recovered. But if it is more than that and we do leave him out this week, we do feel we've got good cover," said Schmidt.

Ireland have three other centres to choose from -- Henshaw's Leinster teammate Garry Ringrose, Bundee Aki and Chris Farrell -- but Schmidt revealed he had requested Will Addison be pulled from Ulster's starting XV as cover, but that he was not yet on his way to Japan.

"That was precautionary just to make sure we have fit and able cover should we need a replacement for Robbie," said Schmidt.

Ireland are hoping to put the disappointment of their 2015 campaign behind them, when an injury-ravaged side were pummelled by Argentina in the quarter-finals.

Since then the Irish have registered two historic wins over world champions New Zealand -- in Chicago in 2016 and at home last November.

New Zealander Schmidt's remarkable six-year tenure, that has included wins over all the southern hemisphere nations, a Grand Slam and three Six Nations titles, will end after the World Cup.

Ireland face a tricky pool containing fellow Six Nations team Scotland and hosts Japan who are desperate to make history by reaching the knockout stages for the first time.

"I think it's a tough order. I think Scotland will be very difficult for us and I think just a six-day turnaround to Japan in Shizuoka will be very tough," he said.

"We're trying to take one match at a time, but we've certainly got an eye to the future and the quality that the Japanese team have," added Schmidt.

In the run-up to the tournament, Ireland put an unconvincing 29-10 home win against Italy and a 57-15 hammering by England behind them to register two narrow victories against Six Nations champions Wales which took them top of the World Rugby team rankings ahead of the All Blacks.

"We know we've been less consistent this year but a lot of what we've been targeting is in the coming weeks," he said.

"So that's really where we know we'll judge ourselves and others will judge us for this year."