It was the fly-half's fourth concussion of 2014 which prompted doctors to rule him out of action for 12 weeks - meaning he will miss Ireland's match against Italy on February 7.
The Racing Metro man will also be unable to take part in crucial club games against Treviso and Northampton but has targeted his country's showdown with France on February 14 as his return game.
"The club would like to have me back for the big European games, and to miss any game for Ireland is a massive blow," Sexton told Irish radio station Newstalk.
"It's what you play rugby for, those big days, to miss that is gutting. I'd like to play but the doctor has my best interests at heart - there are lots of things to consider.
"I'm feeling a lot better - this is week seven of the 12 weeks I've been told to take. I'm feeling better and training harder every week with less symptoms, which is great and exactly what the doctor wanted.
"I'll be mad keen to get back in but I have to take the 12 weeks and take what feels like punishment, but that's the way it's got to be.
"My return-to-play date is February 14, the Ireland v France game, and if I want to be considered for that I'll have to do some contact sessions.
"Even then, I don't know if I'll be considered for that by the coaches, given I won't have played for 12 weeks. I'll let them make that decision. I'll just come into camp in the best shape possible."