McCann came off the bench and tucked away a penalty kick in the 79th minute to earn a point for the Posh.
Ferguson singled out the Northern Ireland international for praise, insisting that the 32-year-old's introduction changed the game for his side.
"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," said Ferguson.
"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 as we don't have a game next weekend. I must thank all the people that helped to get it on."
McCann moved to London Road from Scunthorpe United in 2010.