United are set to face Portuguese giants Benfica in the Champions League at Old Trafford on Tuesday.
Ferguson said: “I have been watching their games, they are playing really well and have improved a lot which is why they are top of La Liga.
“In fact, overall I think they are playing every bit as good as Barcelona and their goals record is phenomenal. And Cristiano Ronaldo, 100 goals in 102 games, is incredible.
“They are definite threats and fit as hell. They really do put some energy into their games.”
