He believed Hart's world-class save from Steven Naismith with the score at 2-1 was the pivotal moment as City went down the other end and added a third.
"There is a moment when Sergio is coming to the side and is trying to come off as he was feeling an injury and the next action he gets into a good position and leads to a goal," said the Spaniard, whose side's Champions League hopes were ended by the defeat.
"We were in total control and for us it was disappointing to concede that goal out of a bit of confusion with a player who needs to come off.
"The first chance of the second half Joe Hart made the save of the season.
"They went to the other end and we conceded a goal in a very important moment - I thought that was the turning point."