Sexton, a key figure in Ireland's Six Nations victories in 2014 and 2015, has only played three times at club level since he was injured on Irish duty in November and failed to complete a full game on each occasion.
Ulster's Paddy Jackson was the only other flyhalf named in the squad for Saturday's opener in Scotland and trip to Italy a week later. The IRFU said on Twitter that Munster's Ian Keatley has trained with the squad and uncapped centre Rory Scannell was also in line to cover.
Flanker Peter O'Mahony, who has fallen down the pecking order after a similarly rough period of injury, misses out because of a hamstring problem while winger Andrew Trimble will be monitored this week with a groin problem.
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Flanker Sean O'Brien and winger Keith Earls, Ireland's other injury doubts, have trained fully this week ahead of Thursday's team selection.
(Reporting by Padraic Halpin, editing by Ed Osmond)
