Johnny Sexton undermined head coach Andy Farrell with his reaction to being replaced in their Six Nations defeat to France, believes Eddie O'Sullivan. 

The Ireland captain was withdrawn at Stade de France with just over 10 minutes to play with his side trailing Les Bleus by eight points. A six-point winning margin would have been enough for Ireland to win the tournament outright. 

Visibly disappointed to be taken off, Sexton, appointed as skipper by Farrell at the start of the Six Nations, shook his head and appeared to look over to where the Irish management team were, indicating that he did not agree with the decision. 

Asked about it in the aftermath, the Leinster out-half said: "I was very disappointed coming off, like everyone would be.

"You're losing the game and you're coming off, so what would you like me to be doing when I'm coming off the pitch? I'm sorry I wasn't delighted coming off."

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Two former Ireland captains, Keith Wood and Brian O'Driscoll, speaking on Newstalk, said that Sexton was out of order.  

Farrell and Sexton are due to hold a joint press conference on Thursday morning with Ireland preparing to take on Wales in the opening Autumn Nations Cup match in Dublin on Friday week. 

"First thing is I wouldn't have substituted him... but that's neither here nor there. That's Andy Farrell's call," O'Sullivan, who coached Ireland from 2001 to 2008, told the RTÉ Rugby podcast. 

[Johnny's] position is under scrutiny in terms of how he reacts to things. 

"For me his reaction was the worst possible reaction. I get he was upset, I get he was annoyed.

"But for the good of the team and to show leadership and captaincy, he has to bottle it up, it's no big deal. 

"You go to the bench, you sit down, you give out to yourself and then after the game you go to Andy Farrell and say 'I want to talk to you'.

"You have your debate with Farrell, he explains his case and that's how you do it.

"You do not throw the coach under the bus, walking off the field. 

"He has to know better than that, he has to know that his reaction is going to be magnified and he has to know that by doing what he did he kind of undermined [replacement Ross Byrne], he undermined the team and he undermined the coach.

"He should apologise to Farrell.

"It was an amazingly poor decision. I kind of think he knew what he was doing, I don't think it was an accident and I think it has damaged his relationship now, whether we like it or not, with the team and with the coach."