Sexton is still lion king
Gatland also looking at merits of Farrell, Russell and Biggar
Warren Gatland has already warned how Jonathan Sexton's durability, or lack thereof, could work against Ireland's out-half when it comes to the British & Irish Lions.
The coach would not have been best pleased to see Sexton leave Stade Antoine last Friday, albeit with a negligible calf strain.
It is just another sign of Sexton's failure to shake off the frustration of injuries in recent years from concussion, shoulder, hamstring and calf issues.
"You want him to get through the Six Nations" Gatland noted when last in Ireland.
"The conversation with Johnny might be like: 'There's been a little bit of concern about your durability.
"'If you do pick up a knock or an injury and you're out for a couple of weeks, the quality of the squad is that, someone else could step into your shoes.
"If you come into a Lions situation, if someone is playing well, you might not be going back into that starting side as a 10, you might be sitting on the bench or not involved.
"How are you going to be able to handle that? What's your attitude around the team and squad going to be like?'"
What will also count against Sexton is the three live and dangerous candidates for the number ten shirt.
Perhaps, it is the one position in the four nations with the highest level of all-round competition.
Sexton would surely be a nailed-on certainty for test starter were it not for his tendency to take on injuries.
The manner of his play against New Zealand in the back-to-back internationals in November showed-up his brilliant game-management when on the field.
However, the fact is Sexton lasted just to the hour in Chicago until his hamstring acted up.
The Australian international was given a miss in order to have him ready to rendezvous with the All Blacks.
This time, Sexton played his part for just 18 minutes before his hamstring strained again.
At the end of the five-week period of strength and rehabilitation, Sexton's calf didn't take too long to cause his removal in Castres.
Owen Farrell, England's inside centre, has a strong mental and physical game which makes him a far more likely challenger than George Ford.
The outstanding form at ten for his club Saracens has been the source of his improvement in the last three years.
Interestingly, Farrell earned a lot from Sexton in his involvement in the 2013 Lions 2-1 test series win over Australia.
"I had a massive respect for him before we even started that tour," said Farrell in 2014.
"I certainly try to do with England as he did with The Lions and does with Ireland."
The versatility of Farrell to play at twelve could be to his advantage and to Robbie Henshaw's disadvantage when it comes to making the first team.
Wales playmaker Dan Biggar has come under increasing pressure from fellow Osprey Sam Davies.
The world-leading execution of collecting his own 'garryowens' has taken the spotlight off the finer details of the position.
He really has to take his game to the next level to seriously challenge.
Gatland's intimate knowledge of the man, not just the player, may work against him.
Glasgow Warrior Finn Russell is probably the most instinctive, off-the-cuff of this elite quartet.
He plays very much in the image of his club coach Gregor Townsend and has been encouraged to trust in what he sees in front of him.
His reluctance to drop back into the pocket against Munster when the game was on the line was either a misjudgement or a symptom of something more significant.
For all his skills, Russell has tended to fade out of the game when he isn't on the front-foot.