Sean O'Brien's injury will come as a 'huge blow' to Ireland's World Cup plans, says former fly-half Tony Ward.

Leinster flanker O'Brien, 32, faces a six-month lay-off with a hip injury.

The absence of the two-time British and Irish Lions tourist means Ireland coach Joe Schmidt must forge a new back-row combination before the tournament.

"O'Brien is one of those players in the dressing room who is a 'go to player'. He's a player you always want alongside you going into battle," said Ward.

O'Brien, who featured in four of Ireland's matches in this year's Six Nations, is the second open-side specialist to be ruled out Ireland's World Cup campaign after his Leinster team-mate Dan Leavy suffered a knee ligament injury in March.

"It is a huge blow. In an area where we have been so strong for so long, with players of the quality of Dan Leavy and Sean O'Brien it is a big loss," added Ward.

"There are still options there, and clearly CJ Stander could be moved from number eight to allow Jack Conan to move into that position and Stander could then possibly play either open-side or blind-side, but no matter how you dress it up it's a big blow losing O'Brien at this stage."

Schmidt has spent much of the four years since Ireland's 2015 World Cup quarter-final exit expanding his pool of players, but the injury to O'Brien is another unwanted problem as the Irish medical staff hope for a positive update on the fitness of line-out specialist Devin Toner.

Ward added: "Joe knows that he can rely on Sean and he plays the type of game that Joe likes to play.

"He's similar to Stander in that he is a very direct ball carrier but O'Brien is a player that Joe has always placed his faith in and this is a setback for Joe and for Ireland.

"Other players will come in but I do think this is a blow at this point in the build up between him and Devin Toner coming off in the Leinster Pro14 semi-final against Munster, they're two big blows to two key players as they begin the warm-up for the World Cup."

A former European Player of the Year, O'Brien has already agreed to join London Irish next season, leaving his international future in doubt because of Ireland's preference to select home-based players - although that policy could change under incoming head coach Andy Farrell.

"Nobody expected him to be ruled out of the World Cup and to be ruled out for a significant amount of time," said former Ireland and Ulster flanker Stephen Ferris.

"From my own personal point of view, you never really know when your last international game is going to be because rugby is such a physical sport and you pick up injuries along the way.

"I finished my international career with a pretty disappointing defeat away to England in 2011 so he'll be disappointed for sure and the whole of Irish rugby will be too because when Sean O'Brien is on his game he's one of the best back rowers in the world.

"Everybody thinks now that he might be past his best with all these injuries but he's a big-game player and Ireland will certainly miss him in Japan."