The Leinster hooker came off with a foot injury in the win over England last weekend. The rest of Ireland's squad traveled to Biarritz, while Sheehan stayed behind in Dublin for rehabilitation.
Ireland's options around the hooker position have also been hit by Rónan Kelleher's hamstring issue.
However, Ireland fans can breathe a sigh of relief because The Irish Times has claimed that Andy Farrell is likely to give a positive update on Sheehan's condition when he announces the Ireland team to play Samoa later today.
Ireland starts their World Cup adventure against Romania on September 9, before playing Tonga on September 16, South Africa on September 23, and Scotland on October 7.
Reports also emerged this week that Sheehan could miss the entirety of the pool stages, but earlier this week, Ireland coach Mike Catt admitted that Sheehan's condition isn't as bad as first feared.
"Yeah, like I say, he's still being diagnosed, so we'll have a much better idea tomorrow or on Thursday in terms of where he's at and when his return dates are," added Catt.
"I think what you get from Dan is one of the best hookers in the world currently, so he's going to be missed in whatever team he plays in. But again, we've got full faith in Rob Herring, Ronan Kelleher, and Tom Stewart, obviously, who is showing his face at the moment.
"So it gives these guys an opportunity to have a pop this weekend, and again, depending on the diagnosis, we'll see how we go on the back end of it.
"Again, Dan is a world-class rugby player. He has been instrumental in terms of how we play our game. So he'd be a big loss to anybody.
"He hasn't been in camp with us; we flew out straight away after the game on Sunday morning. He stayed in Dublin, so we haven't been able to speak to him. Obviously, Andy and the medics are dealing with that side of it. We have a team to prepare, so we're doing that."
Ireland's game against Samoa will kick off on Saturday, August 26 (kick-off 8.45pm local time/7.45pm Irish) this afternoon.
