That was the view of unhappy Minstermen boss Gary Mills after York slipped to a surprise 2-0 home defeat.
It was the first time the Rebels have not lost away from home this season but Mills felt things hinged on Heslop's second booking.
It meant that City had to play the most of the second half a man down - and United capitalised on their numerical advantage.
Mills knows his midfielder didn't get the decision right and reflected after that the red was ultimately what cost his side.
"I think if we manage to keep all of our players on the pitch then we would have taken something from the game," claimed the former Wrexham manager.
"That for me was the difference. We were on top and we were creating chances.
"We may not have been at our best but we still created so overall it's a disappointing result for us.
"We have to produce better and we need to make better decisions also," manager Mills added.
"Simon is very experienced and that was not what we needed at that point in the match, that's for sure."