The Northern Ireland international scored a spot-kick in the 79th minute to earn Posh a 1-1 draw against promotion-chasing Hull.
Ferguson praised the super-sub, and said he was happy to take a point away from the London Road match after Joe Newell had put through his own net to put Hull in front.
"A point was the least we deserved and I wouldn't have been surprised if we had gone on to win after getting ourselves level," he said.
"It was harsh on us to fall behind to a mistake from young Joe, but he is a lad with an excellent temperament and didn't let it affect him.
"The most pleasing thing was seeing our substitutes come on and change the game. Grant in particular upped our tempo, got hold of the ball and tucked away the penalty.
"Home form is crucial when you are fighting to stay up and I know for a fact that the relegation battle is going to go right down to the wire.
"There is no doubt in my mind we have enough quality to stay up but it was doubly-important for us to play today as we don't have a game next weekend.
"I must thank all the people that helped to get it on."