The Red Lichties thought they had cancelled out Adam Matthews' first-half goal when Steven Doris bundled the ball into the net after 65 minutes, but referee Craig Charleston had already blown for a foul on Forster.
And the giant keeper is adamant the whistler called it right over that, but believes allowing the tie to go ahead was a mistake – even though the Hoops won 1-0 to set up a fifth-round trip to Raith Rovers.
Forster said: "It was a foul, 100%. I don't see how it could have been any other possibility than a foul, to be honest. The guy came through the back of me and pushed me when I was about to catch the ball.
"I don't see what the issue is. The referee made the right decision."
Forster continued: "It's a great result for us, but the game should never have been on with the state of the pitch.
"It was frozen solid in the goalmouths, but that just adds to our performance, really.
"We played really well on a really tough pitch. Adam Matthews scored a fantastic goal, and it's job done."
Forster – who made a crucial save from sub Connor Birse when he was clean through in the closing minutes – and the rest of the Celtic players and management, are delighted to have avoided what could literally have been a major slip-up in their quest to win the domestic Treble this season.
"The gaffer set us a challenge to remain in all four competitions going into the new year, and we have done that," said Forster.
"Now it is all about keeping up that momentum, which we are trying to do.
"We are picking up a few results now, and it's important we do that in the league over what is going to be a busy spell over Christmas."
That begins when St Mirren come to Parkhead on Saturday, and Forster hopes they can now rid themselves of the trouble they have been encountering in SPL matches at home.
The irony is that they have managed to defeat Barcelona and Spartak Moscow there, and Forster admits: "I can't explain why this should be the case. It's just not really happened for us at home in the SPL, and when we played Arbroath there in the cup.
"We have picked up some great results in Europe, so it's just about taking that into the league. We have an important couple of home games then we are on the road again.
"We put in a fantastic performance last weekend at Kilmarnock, and won this replay, so it's just important we keep on winning."