Rory sustained the fracture playing for Glasgow in Friday's 20-8 defeat to the Borders at Firhill and will miss the remainder of the Six Nations Championship as a result.
The Lamonts became the first brothers since Gavin and Scott Hastings to pull on the dark blue jersey of their country and both were hoping for a long run together for Scotland.
But Rory has seen Hugo Southwell oust him from the starting XV and Edinburgh utility back Simon Webster keep him off the substitutes' bench - and now injury has robbed him of the chance of putting pressure on that duo for the final two matches with Ireland and Italy.
Sean, one of the stars of Scotland's revival this season, is hopeful the lay-off will prove only temporary, however.
He said: "I'm gutted for Rory because that's two injuries in a row. He did his cheekbone a week ago and thought he was going to be out but then got the OK to play.
"Now he has done his jaw, but it's just one of these things. He hasn't needed his teeth wired up, just a plate in his jaw, so it could only be four weeks rather than months and months.
"It's been nice when we've played together. You always look out for your brother, especially your younger brother (Rory is 23), and hopefully he'll make a speedy recovery and be back in the team at some point soon."
Sean Lamont was the two-try hero against France on the opening weekend of the championship and has proved a danger to opposition defences over the past two seasons with his strong running and devastating pace.
The 25-year-old was, however, hardly seen in an attacking sense in the recent 18-12 victory over England and was forced to put almost all his efforts into the huge defensive effort which won the game for the Scots.
But the Northampton winger insists he will be happy with that again this weekend in Dublin - if Hadden's side rack up their first away win over the Irish.
He added: "It was a little bit disappointing not to get more of the ball in the England game but I'm happy as long as we win.
"I was happy with my defence, the whole team's defence was brilliant, and the main thing was the win - and a win against the Auld Enemy is the one we didn't have.
"It's always good when you come off the back of a win, especially a win against England, but it's back down to business now and we've got Ireland this weekend.
"Ireland are a good team and very dangerous so we have to be careful how we go about this."