Paul Scholes has also raised major concerns about Ole Gunnar Solskjaer's side ahead of Saturday's Manchester derby, where another disastrous result will see the pressure piled back on the boss.
Ronaldo bailed his club out of jail with a stoppage-time equaliser in a 2-2 draw in the Champions League against Atalanta, and may be required to perform even more heroics if the Red Devils are to get anything from City.
Following their harrowing loss to Liverpool, United have shown some character since then with a 3-0 win against Tottenham Hotspur and their 2-2 draw away in Europe as they twice fought back from a goal down.
Despite the mood being slightly better than what it was after the Liverpool match, two of the club's legends, Scholes and Ferdinand, are still extremely worried about a potential repeat against Man City.
And they agree United's chances have taken a hit with the latest injury to summer signing Raphael Varane.
"I don't want to say it again but with the performance like that second half, especially with no [Raphael] Varane, Saturday has to be a slight worry," Scholes told BT Sport.
When asked what City boss Pep Guardiola might be thinking after the Atalanta game, he replied: "You'd think you have a good chance of winning. City are a brilliant football team, brilliant players.
"At any point, whether United have Varane or not it was always going to be a difficult game, but with no Varane now I think it makes it more difficult for United."
It certainly feels like a case of deja vu for everyone associated with the Red Devils as Scholes made similar comments prior to the Liverpool game.
This time however, Ferdinand echoed Scholes' concerns and fears that if his club repeat their performance in the second half it Italy, they will suffer another humiliation.
"They can't have that second half performance," said the former centre-back.
"They go out with that second half performance against City, they get slapped again, three, four or five goals.
"That showed a real lack of respect for the position, being slack, sloppy in situations with and without the ball."
"The second half was a mess," added Scholes.
"We looked at one point, what formation where they were playing? It was disarray, it was just hope.
"Again it comes down to an individual, a moment of brilliance and that man Cristiano Ronaldo.
"He saved them in that second half."
talkSPORT will bring to LIVE and EXCLUSIVE commentary of the Manchester derby, Manchester United vs Man City, on Saturday - kick off 12:30pm