The inspirational form of Danny Collins alongside Nyron Nosworth in Sunderland's central defence means Ferdinand cannot be guaranteed a place when he is fit enough to make his Black Cats debut.

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Ferdinand, who has penned a four-year contract after switching from the Hammers for an undisclosed fee, believed to be around £8million.

The centre-back is recovering from a hamstring strain which will keep him out of action until the September 13 trip to Wigan.

The 23-year-old said: "It was hard leaving West Ham because I have been there for 16 years but, after meeting Roy Keane, I knew it was time for a change.

"This is a great challenge for me and I feel it is an exciting time at Sunderland. I think the move will do me good."

Keane believes joining Sunderland can help the ex-England Under-21 cap fulfil the potential he showed at Upton Park.

Keane believes he can get the best out of the Londoner who has become his ninth summer signing, following deals for Teemu Tainio, Pascal Chimbonda, Steed Malbranque, Nick Colgan, David Meyler, El-Hadji Diouf, David Healy and Djibril Cisse.

Keane said: "We needed to strengthen our defence and Anton is a great signing. He'll bring quality and competition for places which is what we want, hopefully we can take him to another level here at Sunderland.

He has come through the ranks at West Ham and maybe just needs a change."

Keane still wants to add to his squad before the transfer window closes on Monday, with a left-back the priority.

Keane explained: "I have one more player in mind and then that is the end of it for a few months at least."

As Ferdinand departed West Ham, Hammers boss Alan Curbishley said: "I wish Anton well in the future. There has been a lot of speculation over the transfer but the bottom line is that we have a lot of defenders at the club and the circumstances around the deal mean it was the right decision.

"As I have said previously, I am confident we have a strong enough squad and this will especially encourage James Tomkins to show me what he is capable of doing. I am also looking to bring in one or two players to go with Jan Lastuvka and Valon Behrami who we have signed this summer."

West Ham chief executivew Scott Duxbury added: "I would like to wish Anton all the best in the future and thank him for his loyal service.

"We know that our fans have a particular affinity with players that have come through the academy system, but we think this should help us to promote even more young talent through the ranks.

"This deal was also the right one for the club, given we could not agree a new contract and that there were just two years left to run on the existing one."