Johnson will be in charge for the Scots' RBS 6 Nations opener with England on February 2 and his first major decision will be deciding which one of three players takes over the 85-time capped Brive half-back Blair following his decision to step down from the international game on Monday.
He told Press Association Sport: "When you lose a bloke with 80-odd caps, it is not easy to replace him. But if Mike thought it was the right decision to retire, it was the right decision. It causes us a bit of grief but the reality is that we have to move on now. We have to find out who that next player is for the number nine jersey."
Among Johnson's options to take over from Blair - who has played more games for Scotland than any other scrum-half - are Edinburgh stand-off Greig Laidlaw, who has operated at number nine for Scotland in the past, or either of Glasgow duo Chris Cusiter and Henry Pyrgos.
Cusiter, however, is still recovering from a shoulder injury while 23-year-old Pyrgos has just three caps to his name. Johnson is not overly worried by the prospect of any of the three taking Blair's old spot.
The Australian, who has previous Six Nations experience following a similar caretaker role he enjoyed with Wales, said: "There has been some good signs from the boys ready to come in, and who's to say that the next guy won't become the most special nine that Scotland has ever produced.
"Greig will be there or thereabouts but there are others too. We are waiting on the injury update on 'Cus' and we will see how Henry Pyrgos is and the form of others as well. We will go with what we have got. That is all we can do."
Johnson replaced Robinson following Scotland's abysmal 21-15 defeat to Tonga at Pittodrie at the end of November. He had originally joined Robinson's backroom staff as assistant coach and admits a decision on whether his new post will be made permanent will have to wait until after the Six Nations.
He said: "How long will I be here? Well that's two-fold. It's not just my wish, it comes down to those of others. I came here to do a certain job and that is still my mandate. But the circumstances have changed and I have agreed to do a different job until a certain time.
"However, my other job still exists and I will re-evaluate my situation with the board at a later date. I will make my decision in the best interests of what Scotland needs. If the board decide I'm the person they want and if that suits me, I will review it then."
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