The British and Irish Lions fly-half could be sidelined for two months as a result of the injury, which was caused by a collision with Craig Burden in a Top 14 victory over Toulon on Saturday.
The 29-year-old faces a race to recover in time to be in contention for a place in the three-Test autumn series, which starts with the visit of South Africa to Dublin on November 8. The Irish then host Georgia and Australia.
Racing overcame the French and European champions Toulon 17-10 in Paris at the weekend, but Sexton was withdrawn after an early clattering from Burden. The South African hooker was sin-binned as a consequence of the challenge.
Sexton's injury will be a sore point too for Irish rugby. The fly-half is expected to complete a move back to Leinster next summer in an effort to boost his country's World Cup build up. The 45-times capped playmaker is arguably Ireland's most important backline player following Brian O'Driscoll's retirement and so will be afforded every opportunity to regain his fitness in time to play in the autumn.
Sexton had been a serious doubt for the autumn series last year after succumbing to the heavy workload expected of him after first joining Racing, with the Irishman suffering a leg injury. Should he miss out this year then Ulster's Paddy Jackson is expected to replace him in the fly-half role, although Leinster's Ian Madigan and Munster pair Ian Keatley and JJ Hanrahan would also be worthy of consideration.