Byrne in line to get a shot at Twickenham
There is no guarantee Joey Carbery will travel to the World Cup.
And there is none that Jonathan Sexton will start against England at Twickenham on Saturday (KO 3.0, Sky Sports Action).
The reigning World Player of the Year sat out training at Quinto do Lago yesterday after two hard sessions over the weekend in Portugal.
This means an opening of the door to Jack Carty or even Ross Byrne.
Sexton was joined in a watching brief from the sidelines by centre Robbie Henshaw, wing Keith Earls and second-row James Ryan.
Murphy was quick to quash any nervousness over the quartet's availability for Japan.
"There's no real fears about them," said Ireland assistant coach Richie Murphy.
"It's just about managing them and getting their work-load right for possibly involvement this week and, if not, the following week."
However, the current uncertainty over Ireland's first and second-choice out-halves has caused a change of plan for the next three matches.
Connacht's Carty is best placed to take advantage of the misfortune to Carbery and even Sexton.
Leinster's Byrne, long considered a long shot, could even grab a lifeline in London.
As regards Japan, Carbery had a minor procedure to the joint of his ankle shortly after suffering an injury against Italy.
The four-to-six-week timeframe for the 23-year-old's return to full fitness will have to go to plan. At the moment, Carbery is hitting all his markers. He just cannot afford a setback of any significance.
"He's recovering really well. He's well into his recovery programme and, talking to him, he feels good and confident," issued Murphy.
"He is exactly where the medics would expect him to be at the moment."
Somewhere along the line, the coaches will need to know whether Carbery will be ready for the opening test of the World Cup on September 22.
"He's definitely trying to get back for that first Scotland game," added Murphy.
"It's a case of how we manage that and also manage the squad. Although he is very much online for that time-frame, we also have to have contingency plans in place.
"We'll look at them through the next few weeks and get game-time for certain people."
This is the sort of message that Carty and Byrne will take encouragement from, at least in the short-term.