United continued from were they left off at Old Trafford by dominating the game but had to come from behind to gain a valuable Group E Champions League point in a 1-1 draw.
Then for 71 minutes it was a brave rearguard action by the Scottish Champions until six minutes from time.
Artur Boruc failed to deal with Ronaldo's free kick and Giggs pounced to equalise.
The Reds almost snatched a deserved winner but Dimitar Berbatov shot horribly wide from eight yards.
"In fairness we kept our perseverance and kept on going at them and eventually deserved our point.
"But with minutes to go you think you are going to lose it and it's not going to be your night – so it's a good result for us.
"Credit to the players for a very good second half performance."
Ferguson reserved mention for the crowd.
"We spoke before the game of trying to quieten their support down, but they are a fantastic support and keep the team going. It gives them the energy to chase and press the ball and that's what Celtic were tonight a very industrious team and not easy to break down.
"The second half we created some fantastic openings by getting to the dead ball line and that's what we had to do against such support."