Rafael da Silva believes Manchester United could be destroyed by Manchester City in Sunday's derby if they continue to defend in the sloppy fashion that has caused Sir Alex Ferguson's side to concede first in 15 of their 23 games this season.

United travel to the Etihad Stadium with a three-point lead in the Premier League. On Tuesday Ferguson compared the defending to the Cartoon Cavalcade children's programme and expressed concern that it could be costly against City.

Da Silva, asked if he agreed that Roberto Mancini's side could destroy United, said: "Of course, yes, if we defend like we have done in some games we will be in real trouble against a team like City.

"That's 100% sure. I don't know why we have had problems defending this season. If we work hard in training I'm sure we can learn to defend better.

"In the Premier League Manchester City are doing very well – they just seem to keep going and going. But we just need to concentrate on ourselves. It is a big game for us and we need to play at our best level. They have a very good attack with some quality strikers. This will be our biggest test of the season. They have two Argentina strikers [Carlos Tevez and Sergio Agüero] and I am from Brazil so that increases the rivalry for me."

Last season City won both derbies, recording a 6-1 rout at Old Trafford in October and a 1-0 win in April's return match at the Etihad. "The derby is a big game and it is getting more and more important each season," said Da Silva, who knows victory would double United's advantage over City as the busy festive programme beckons.

He said: "It would be very important to build a six-point lead over them before Christmas. That would make them nervous and make it easier for us. We are really determined to make up for losing the title to them last season. The way we lost it [on goal difference, on the final day] made it even harder to accept. This season we are determined to win the title back. I will never forget how I felt that night after we had lost it and that will give us more motivation, definitely.

"We just looked at each other and said: 'How did we lose it by a few goals?' It was hard to take. It took ages to get over it – and it [the pain] is still there."

While the 22-year-old has seen his twin brother, Fábio, loaned to Queens Park Rangers, Da Silva has established himself as Ferguson's first-choice right-back, the Scot regularly praising his development. "It's been a good season for me personally," he said. "I have played most of the games at right-back and that is good. Everyone wants to play games. I just hope I can keep it going."

Ferguson believes that if United can triumph at City this would represent one of the club's finest victories. "If we win it will be one of our best ever results," he told MUTV. "They are a really good, powerful team with massive players. It won't be easy and if we defend like we did at Reading [winning 4-3] we'll be in trouble. The important thing for us is to take lessons from the mistakes we're making and do something about it. We need to find a solution."

Ferguson, who may be without Tom Cleverley due to a calf injury picked up in Wednesday evening's 1-0 defeat by Cluj, believes that Chelsea are not yet out of the championship. "At this time in the season you can't really say it's a two-horse race," he said.