Chris Henry will mess with Scotland's minds at the breakdown in Ireland's RBS 6 Nations opener, according to his back-row colleague Peter O'Mahony.
Leinster battering ram O'Brien is unlikely to play again this season after undergoing shoulder surgery.
And Munster's determined blindside dynamo O'Mahony has backed replacement Henry to cause Scotland serious headaches over the ball this weekend.
"Chris is an out-and-out openside; he's dogged, he's at the coal face as we like to call it, at that first breakdown he's causing havoc," said O'Mahony.
"He's a class footballer, so I don't think there's any worries of what he'll bring to it.
"He'll wreck your head if you let him, and he'll take that as a huge compliment I'm sure."
Talismanic lock Paul O'Connell assumed Ireland captaincy duties in the autumn Test series under new boss Schmidt.
O'Mahony has total faith in his Munster team-mate's stewardship, but challenged his Ireland colleagues to assume more personal responsibility than in the past.
"These weeks guys really have to worry about getting themselves right," said the 19-cap 24-year-old.
"We're looking to get to the stage where we have 10, 12, 15 leaders on the pitch.
"If we're going to be competing for Grand Slams or Six Nations we've got to have everyone leading the charge, and everyone's got to see themselves as a leader in their own right."
Ireland forwards coach John Plumtree admitted Scotland boss Scott Johnson sprung several surprises with his pack selection.
Plumtree said Ireland will not alter their approach to accommodate unexpected selections like lock Tim Swinson.
But he did concede he has warned his pack to expect the entire back-five of Scotland's pack to aim to play like loose forwards.
"They've got some quite good depth there," said Plumtree.
"Their pack was slightly different from what I thought it would be.
"But we don't really have to change too much, because we know they will be coming hard at us with the ball, and trying to make it a real battle at the tackle.
"They've got a dynamic back-row and two gritty guys in the second row there too, so we'll have our work cut out.
"Tim Swinson is a big carrier, and a pretty physical tackler, so he can cause problems if allowed.
"And then they have Richie Gray to come off the bench and he will add an extra element in the last 20 I'm sure."
Former Wellington Lions and Natal Sharks coach Plumtree admitted fans' fervour in the Six Nations build-up had caught him slightly unawares.
Buoyed by that extra backing though, he vowed Ireland are determined to match that zeal when Sunday's kick-off finally arrives.
"It's been a long time since our last performance, so it's just great to get back into it," he said.
"It seems as though supporters hype it up more than Southern Hemisphere Test matches and Super 15 rugby.
"I'm sure the intensity of the occasion will match that: we'll definitely be ready."