Sir Alex Ferguson has admitted Nemanja Vidic might still not be ready to make his first start since September when Manchester United visit Swansea on Sunday.
The United captain made his comeback as a substitute in last week's 3-1 win over Sunderland after three months on the sidelines with a knee problem. But Vidic, who suffered a cruciate knee injury last December, has only started four league games in a year and Ferguson is reluctant to rush him back into the team.
He said: "Vidic might not be ready to start. Vida is doing great in training, it's just a matter of picking the time to bring him back."
United will definitely be without Rafael da Silva at the Liberty Stadium, meaning Chris Smalling and Phil Jones are potential deputies, while Jonny Evans, who missed the Sunderland game because of a hamstring injury, is available again.
"Rafael is still out with a hamstring injury," Ferguson added. "But I have options [in the centre of defence] with Smalling, Jones, [Rio] Ferdinand and Evans."
United will still be without injured winger Nani and midfielder Shinji Kagawa, who has been out since October with a knee injury, is due to return to first-team training on Monday.
The trip to the Liberty Stadium will be the first of five games in 14 days for United and Ferguson has said he will rotate players as he plans to maintain United's six-point lead at the Premier League summit.
He added: "Utilising the squad will be important, as will picking the right team, but hopefully we can get through it and be in the same position as we are now."
Ferguson also praised Swansea, who are in the upper half of the Premier League, for their passing football as well as their recent success.
He said: "You've seen the results they have had and the progress they have made as a football club over the last years. The important thing for me is they try to play and have the courage to play their football, which is good."