Leinster will be able to call on Jamison Gibson-Park, Dan Leavy and Rory O'Loughlin for Saturday's Champions Cup clash with Bath at the Aviva, while the game comes too soon for Johnny Sexton.
The out-half was ruled out of action for between four to six weeks after twisting his knee and ankle during Ireland's Autumn Nations Series match on 13 November.
The 35-year-old hopes to figure in round two against Montpellier on 17 December.
"I think the Bath game will probably be a bit too soon for me, but if I can get back for the Montpellier game, that's the goal at the moment," he told reporters.
Flanker Will Connors is also out of the opening games with "a hamstring injury picked up in training last week."
Lock James Ryan, prop Cian Healy and back row Jack Conan appear in the 'further assessment required' section of the medical update.
"Cian Healy was withdrawn with cramp at half-time against Connacht Rugby as a precaution and will be further assessed this week," it read.
"James Ryan will continue to train with the team and in line with World Rugby protocols James will continue to be monitored including seeing an independent concussion consultant as part of his return to competitive action.
"Jack Conan will look to increase his training load this week and will be further assessed before a final decision is made on his recovery from a quad injury."
Meanwhile, scrum-half Gibson-Park (thigh), Leavy (concussion) and O'Loughlin (shoulder) have all returned to full training and are available for selection for Saturday's Pool A game (3.15pm).