Leinster flanker Josh van der Flier said on Tuesday they will miss the "asset" of Johnny Sexton after the Ireland fly-half was ruled out of this weekend's European Champions Cup semi-final at La Rochelle with concussion.
Sexton, 35, who skippers the Irish province and his country, suffered the head knock during the quarter-final victory at holders Exeter Chiefs on April 10.
Van de Flier backed 26-year-old Ross Byrne, who started March's Pro 14 final win over Munster, to replace 105-Test stand-off Sexton.
"Johnny's obviously one of the best players in the world. He's an asset to the team. We would all like to have him playing but Ross Byrne has come in for him and has been brilliant," van der Flier said.
"I have no doubt he can fill his boots. He's been incredible for us all year so although we'll miss Johnny we have faith in Ross," he added.
In 2014-2015 Sexton was sidelined for 12 weeks after sustaining a head injury while playing for French club Racing 92.
Elsewhere, Leo Cullen's men could be without Test back-rower Caelan Doris as well as Ireland scrum-half Jamison Gibson-Park, who also have fitness issues ahead of Sunday's trip to the Atlantic coast.
La Rochelle host the joint record four-time Europan champions in the French outfit's first appearance in the last four.