Ireland, in a four-way tie at the top of the standings but with a superior points difference, host Italy on March 8 before travelling to France a week later where a win could seal their second championship in 29 years.
Sexton, Ireland's linchpin at number 10 who missed most of last year's poor campaign through injury, returned to Paris with his thumb in a splint following last weekend's bruising 13-10 loss to England.
"Jonny Sexton came back from Twickenham with thumb ligament damage. He will be out of action for 10 days to six weeks," Racing Metro coach Laurent Labit was quoted as saying on the team's twitter page.
Ulster's Paddy Jackson and Leinster's Ian Madigan, both set to feature for their provinces this weekend, will vie for the number 10 shirt if Sexton is ruled out.
In better news for Irish coach Joe Schmidt, British and Irish Lions winger Tommy Bowe will start for Ulster for the first time since November on Friday to put himself back in contention for the remaining Six Nations matches.
(Reporting by Padraic Halpin; Editing by Rex Gowar)
