Ferdinand is currently enduring some challenging times with Manchester United as they experience a bad run of form in both the Premiership and the Champions League.

The former West Ham and Leeds defender was substituted during the 4-1 defeat against Middlesbrough last weekend and it was thought his position in the England side could come under threat after Jonathan Woodgate was recalled to the squad for next week’s friendly against Argentina.

But Eriksson is adamant that Ferdinand is still a key player in the England team and will be an influential figure at the World Cup next summer.

Eriksson said: "Rio is one of the best central defenders in the world. Maybe he is not in the best form at the moment which happens. I don’t know why that is because I have not spoken to him about it.

"If you take the last World Cup I think he was the best defender in the whole competition. He is still young, so I cannot see any reason why he should not reach that level again and he will do that. I am not worried about Rio Ferdinand.

"In this case I will pick a player on past form because I know what Rio can do. He can be a very important player in the squad.

"It is the same when it came to whether Frank Lampard and Steven Gerrard should have been picked to play at the start of the season.

"We all know what those kind of players can do. I am very happy that I have stayed loyal to the players."

Eriksson reveals he held talks with Ferdinand before he dropped him for the World Cup qualifier against Austria last month as he opted to play John Terry alongside Sol Campbell at the heart of the England defence.

But Campbell suffered an injury in the second half which allowed Ferdinand to feature in the final qualifier against Poland. And the Manchester United defender is now hoping to keep his place for the forthcoming friendly in Geneva next Saturday.

Eriksson added: "I talked to Rio before putting him on the bench and he was fantastic. He said that is football sometimes and he knows what to do to change it.

"For me Rio is the most complete central defender we have in this country. He can do everything and can play football.

"Rio has got 42 caps and I can’t remember one of those games when he played badly. He was not the reason why we lost 4-1 against Denmark.

"I have never said he will be my first choice central defender because we still have Campbell, Terry, Woodgate, King and Carragher. But it might be that all of them will go to the World Cup."

Eriksson is looking forward to having Woodgate back in his squad again as the Real Madrid defender has now finally recovered from a string of injuries which have affected his career over the last 14 months.

Woodgate’s last game for England was against Sweden back in March 2004, but the England head-coach is now hoping he can regain the form which made him one of the best defenders in the European game.

Eriksson said: "Woodgate is finally starting to play football again after two-years out. He is doing very well. I have seen him three times on television, but have not been to Madrid yet.

"But the reports I have and I have talked to [Arrigo] Sacchi at Madrid - he is doing very well. It is a good opportunity for him and for me to come into the squad again and see what he can do.

"He is somebody that I want to see close-up. I remember the few times he played for England that he did very well.

"He is a very good defender. He is elegant, quick, a mobile central defender and can play football. The competition to play in central defence is very difficult at the moment, but I’m sure he would have had more caps if he was not injured."

Eriksson insists he will treat the friendly game against Argentina as a competitive match and hopes it will provide his side with a stern test as they countdown to the World Cup in Germany.

And after gaining qualification for the finals last month, Eriksson is already predicting that England stand a one in seven chance of lifting the World Cup next summer.

He said: "I want to play my best team against Argentina. I will treat it as a competitive game. This is a very good test for us. We are playing one of the best teams in the world and we have good preparation time.

"I have always said that we would go to the World Cup and we would do very well. I still believe that. Of course, it's my dream - as it's yours - to win it. We will try to do that.

"I think we are one of seven teams that could do it along with Brazil, Argentina, Germany, Holland, Italy and France. Maybe Spain and the Czech Republic have a chance if they qualify, but I think one of those seven will win it."

Sven's squad for the match against Argentina is named here on TheFA.com on Sunday evening.