The Pars were due to head to Ibrox on Saturday, October 12 but the Gers asked the SPFL to call the game off due to international call-ups.
Lee Wallace, Arnold Peralta and Lewis Macleod have all been selected for Scotland duty while Andy Little and Dean Shiels could yet feature for Northern Ireland.
Fans reacted with fury at the decision to postpone the game - which has been rescheudled for Wednesday, November 6 - but McCoist insists it is the right thing to do.
He said: "It was my opinion that we should get the game cancelled, 100%. I will explain why.
"I can totally understand the fans' frustrations. It is my job to look after the football team and I am not looking after the football team if we don't do what is best for the football team.
"It was my opinion and my staff's opinion, and we had a good, long chat about it, that we should cancel the game.
"We could possibly have 11 or 12 players away. It is not as clear-cut as you think. The certainties are Peralta, Wallace, Macleod. There is every chance Shiels and Little could be away and I think we have seven possibilities in the Under-19 team.
"Gasporatto is away and you have got Aird and Murdoch, guys that have a chance of being on the bench, if not starting.
"I don't believe we are doing our job if we take on anybody, never mind the second-best team in the league, with a completely under-strength Rangers side.
"You then open up the argument that they should be able to beat so and so with the squad we have got, I can understand that.
"But can you imagine the press conference we would be having if Dunfermline beat us 2-0 at Ibrox and we were without eight players.
"You would, quite rightly, be asking me if I was doing my job correctly."
McCoist's side will play their final fixture before the international break at Ayr tomorrow, with Richard Foster a doubt for the Somerset Park clash.
Midfielder Kyle Hutton is fit after an ankle injury and will be in the squad.