Both teams arrive in Spain on the back of impressive European campaigns to date, but each will be forced to play the semi-final without one of their most influential players.
Veteran French centre Traille injured his right forearm in the Top 14 clash with Clermont Auvergne last weekend and could miss the rest of the season.
In what is clearly a massive blow to the Basque giants, the 30-year-old is expected to be out of action for between four and six weeks.
Meanwhile, 2009 Lions leader O'Connell has failed to recover from a groin injury that has seen him sidelined since Ireland's Six Nations defeat to Scotland more than a month ago.
Munster head coach Tony McGahan revealed that the imposing second row would be in the frame for selection if he returned to full training this week but that hasn't happened and O'Connell therefore won't be named in the matchday squad.
But while the absence of the man who has played six straight Tests for the Lions will obviously be hugely disappointing for Munster and their Red Army of supporters, Barnes, Morris and Evans feel it is Biarritz who will struggle to cope most with replacing a key figurehead in their starting line up.
Traille was the standout performer for Biarritz in their quarter-final win over the Ospreys earlier this month, scoring three drop goals and marshalling his side's midfield defence and attack.
"Traille will be missed more," Barnes told the Rugby Club programme.
"That's no disrespect to Paul O'Connell but we saw against Northampton that Munster can get through without O'Connell now.
"Damien Traille's ability to boot the ball 60 metres and just get Biarritz in a position to drop goals is crucial.
"In a tight game, and you expect a semi-final to be tight, Traille's ability to kick the ball 60/70 metres and then take drop goal after drop goal successfully can win a game."
Both Morris and Evans also highlighted that ability to drop goals at critical stages in a match as one of the major reasons why Biarritz could be feeling Traille's absence so acutely come the end of this weekend's titanic battle.
"Keeping the scoreboard ticking over is a very simple policy used by most French sides, particularly Biarritz," explained former England and Lions scrum-half Morris.
"They're very strong in the pack and, when they're in the 22, they come away with points.
"You build pressure and that's what Traille does."
"We've said that French teams have a fixation with drop goals - Mr Elissalde at Toulouse has a similar fixation- but those drop goals (against the Ospreys) were all bang on: the right thing to do at the right time," added Evans, a hero of the 1989 and 1997 Lions tours, as well as a Test Lion in 1993.