Roy Hodgson was last night left to contemplate a worrying shortage of experienced international centre-halves for England's two forthcoming World Cup qualifiers, with Gary Cahill a serious doubt due to injury compounding Rio Ferdinand's decision to withdraw from the squad.

There was consternation at the Football Association's joint announcement with Ferdinand, that the Manchester United player had met with Hodgson on Sunday night and both had decided that the 34-year-old defender would not be able to tailor his strict training programme to accommodate the qualifiers against San Marino on Friday and Montenegro a week today.

That was in spite of the fact that Ferdinand was only likely to have been asked to play in the latter of those two games. Starting on 30 March, a week on Saturday, United play Sunderland, Chelsea in the FA Cup sixth round replay and then Manchester City in the space of 10 days, and it would appear that the defender is slated to figure in all three games.

As for Cahill, he had a scan last night and will be assessed by the FA's medical staff this morning ahead of training in the afternoon. The Chelsea defender picked up a problem against West Ham on Sunday having landed awkwardly on his knee after jumping for a header.

Cahill is by no means certain to be fit and Chelsea will be mindful of their punishing fixture schedule, including that FA Cup replay against United. Tottenham defender Steven Caulker was promoted from the Under-21s squad yesterday to replace Ferdinand, the second late call-up for a centre-half after Joleon Lescott. With Michael Dawson and Ferdinand now out of contention, and Cahill carrying an injury, Hodgson has just Lescott, Caulker and Chris Smalling fit among his centre-halves.

There is the possibility that Michael Carrick could be co-opted into the role, as he has done for Manchester United in the past. The centre of defence will be a concern for Hodgson in Podgorica. The Montenegro coach, Branko Brnovic, has named both Mirko Vucinic of Juventus and Stevan Jovetic of Fiorentina in his squad for the team's forthcoming games against Moldova and England.

The announcement on Ferdinand was made yesterday lunchtime after a meeting between Hodgson and the player at a hotel in London on Sunday night. Ferdinand later tweeted that he was "gutted" his pre-planned programme" of treatment and training could not be adapted in order to let him play.

Hodgson did not rule out selecting the player in the future and Ferdinand has said that he too would like to make himself available for the national team. The England manager said: "I'm disappointed Rio will not be available, but due to the detailed pre-planned training and medical programme he must follow it's not possible.

"However, I was pleased that Rio called and asked to meet with me. It was important to hear from him personally about the way he must manage his body between games.

"This is not to say he cannot play back-to-back games – he can and has proven so. He's out this time due to particular pre-planned details already in place for his programme.

"I must place on record how I was impressed with his commitment to playing for England and I look forward to hopefully selecting him for squads in the future."

In a statement published on the FA website and Ferdinand's Facebook page, he said: "It was important for me to be able to speak with Roy face to face and explain the situation, and one thing I made clear was that my passion and commitment to represent my country is as strong as ever.

"It is disappointing that I won't be able to play a part in the upcoming games but I told Roy that I want to continue to be available for England and I look forward to working with him in the future."

The England squad convened last night at St George's Park, the FA centre in Staffordshire, and will train for the first time this evening ahead of the World Cup qualifiers.

Rio and Roy: key to the future

Rio Ferdinand

"It was important for me to be able to speak with Roy face to face... I told Roy that I want to continue to be available for England and I look forward to working with him in the future."

Roy Hodgson

"I must place on record how I was impressed with his commitment to playing for England and I look forward to hopefully selecting him for squads in the future."