Ronaldo is the leading scorer in the European club competition this season with seven goals so far.

And in his column in Catalan sports daily El Mundo Deportivo on Friday, Ferrer conceded that stopping the Portugal winger is easier said than done.

"Defending against Ronaldo is almost like a lottery," the former defender said.

"He doesn't normally ask for balls in the hole and the best thing is to be on top of him so he doesn't receive the ball, marking like Dani Alves does, putting the foot in constantly.

"If you give Ronaldo space, you are dead. He has evolved and when he shapes to dribble, he can surprise you with a cross. He is almost unstoppable."

Ferrer also added that Barcelona must try to push Ronaldo and Nani back into more defensive positions.

"One solution would be to make Ronaldo and Nani work by trying to get (Gianluca) Zambrotta and (Eric) Abidal forward and moving the ball around with patience," he added.

"Ronaldo's verticality gives United a sixth gear; they are the only team in Europe which has that."

Ferrer is also impressed with Ferguson's array of options in midfield.

"Ferguson can opt for a defensive-minded or attack-minded midfield," he continued.

"If he (Ferguson) wants more of a footballing midfield, he can play Scholes and Giggs (with Ronaldo and Owen Hargreaves) and if he wants to strengthen it (defensively), he can choose Carrick alongside Hargreaves.

"On the left wing, the doubt is whether Nani or Giggs will start. The former is more explosive and fast, closer to Cristiano, but the Welshman brings experience, vision and goals, as does Scholes," he added.

Ferrer, who played 204 games for Barcelona between 1990 and 1998, also highlighted Wayne Rooney as a big threat to his former club.

"The (United) system is really a 4-4-1-1 with Rooney the furthest man forward," Ferrer said.

"The number nine is a real show, a goal-poacher and a good ball-winner. Maybe he abuses that (quality) and drops too often into midfield, but his power and his shots are brutal.

"Rooney unsettles the rival defence by supporting the midfield. The centre-back who doesn't follow him should stay as a libero."

Despite praising United's attacking attributes, Ferrer identified the right-back position as a potential problem for the Red Devils.

"On the right (of the defence), there is a small problem (for United)," he explained.

"Gary Neville is still not right and (Wes) Brown is more of a centre-back who offers little on the right. (Lionel) Messi would find it easier to attack Brown than Patrice Evra," he said.

Ferrer, himself a right-back with the Catalan outfit and later Chelsea, believes United have few other noticeable weaknesses at the back.

"If (Nemanja) Vidic recovers, his tandem in central defence with (Rio) Ferdinand is one of the best in Europe.

"Although the latter is not that quick, the Serb covers that weakness. Both are very good in the air and complement each other at the back in a very compact central defence.

"Evra is a great left-back with a similar style to Sylvinho. He is quick, recovers (possession) very well and also moves into midfield well," he said.