United may be struggling to convince just now, and defended poorly during their surprise home defeat to Tottenham on Saturday.
Their European reputation also took a hefty blow with last season's Champions League group stage exit.
However, for a team like Cluj, they remain a club apart.
And Cadu underlined just what underdogs he regards his team.
"They are the best opponent we have faced," said the 30-year-old defender.
"They have a 90% chance of winning. We have the other 10%."
Cluj coach Ioan Andone has encountered United before.
The one-time stylish defender was in charge of Dinamo Bucharest in 2004 when the Red Devils recorded a 5-1 aggregate win against them in the final qualifying round.
Andone has moved on plenty of times since then in a coaching career that began seven years after Ferguson arrived at Old Trafford and has involved 17 different stints at various clubs, some of whom, including Cluj, he has managed more than once.
"Who wouldn't envy a career like Sir Alex Ferguson's," said Andone.
"He has been able to stay so many years at the same club, winning trophies and raising and promoting players to the club.
"It is something you cannot think about in Romania."
Winning the title at Dinamo in 2004 remains the pinnacle of his career to date.
Yet, despite Cadu's pessimism, if victory can be achieved on Tuesday, it would probably eclipse that.
"The prestige would be a big thing first of all," said Andone.
"Secondly it would give us six points in the group.
"But for the club, the players and the importance of the entire region of Transylvania, it would be phenomenal."