Barry was convinced the trophy was being transported up to Wearside where United were winning 1-0.
He was replaced by Edin Dzeko 21 minutes from the end of yesterday's [Sunday] final game against QPR, with the Blues losing 2-1.
But in stoppage time Dzeko and then Sergio Aguero netted to turn the entire campaign on its head and end City's 44-year wait for a league title.
"At 2-1 down I was thinking this is done," Barry said.
"I looked up and saw a helicopter and wondered if it was the main trophy being flown north.
"Towards the end, I was sitting there watching our fans leaving, just thinking 'Here we go again'.
"In a way, it makes it more special but I would have preferred an easier win."
Barry said: "It was only one or two leaving, not many, and if it had been a normal game midway through the season I'm sure there would have been a lot more.
"But I'm sure most fans realised that this season has been something special, and that there was something extra special stored up for the end.
"But that's football, you never know what's going to happen, and that finish was something else."
The scenes at the end will never be forgotten though, with Barry amongst those racing off the bench in jubilation as Aguero was smothered by ecstatic team-mates.
"Those last five minutes were incredible," said Barry.
"I can't really find the words to describe what happened.
"The way it finished was unbelievable. This season has been so up and down that it was no surprise that the final finish went that way.
"But I don't think that will ever be beaten."
The agony for Barry was made all the more acute because he was a just as helpless as the fans who had spent more of the afternoon chewing their fingernails, desperate for their team to deliver the prize that had eluded them for so long.
Yet, out of the 90 minutes, City spent 61 of them trailing Manchester United in the 'as it stands' table, which just made the day even more memorable for Barry.
"I will remember it well because I had to sit and watch, and that is probably worse," said the England midfielder.
"You would rather be on the pitch. It's far more nervous to watch because you cannot affect it.
"But I have soaked it all in, because you have to enjoy moments like this - it's been fantastic so far."
The England international said: "You can't hold yourself back when something like this happens, especially in the way it happened.
"Even people who are grounded and don't normally show their emotions weren't able to hold themselves back.
"It is a part of history that may never be beaten, that sort of finish.
As news filtered through that Stoke had held Bolton to a draw at Britannia Stadium, the need for Rangers to claim even a point from the game was gone.
Barry, 31, said: "I think that helped us. Once we got the equaliser, their fans started celebrating and there signals going onto the pitch that they were safe anyway.
"Whether that helped us or not I am not sure but we are delighted if that was the case.
"Whatever happened we had to find another gear and we managed to do it."
United finished at the Stadium of Light believing they might have won the title but facing an anxious wait to hear the final score from Eastlands.
But the five minutes stoppage time played - which was partly due to an extraordinary fracas in which QPR's former City midfielder Joey Barton was sent off - prolonged their agony before the bad news came through.
Barry said: "I heard United were watching us on the screen for the last couple of minutes and saw us score the winner - whether that's true I don't know.
"Once they had heard it was a 2-2 scoreline I am sure it was something they weren't expecting anyway, and you can understand it if they were celebrating."