Manchester United boss Sir Alex Ferguson admits Nemanja Vidic and Rio Ferdinand may struggle to play together again.

Vidic, 31, was omitted from Fergie's line-up for the Champions League draw against Real Madrid on Wednesday after playing a full 90 minutes the previous Sunday against Everton.

And boss Fergie revealed: "I had my doubts about whether I was doing the right thing or not because he was fantastic against Everton, but I stuck by what we've been doing with him.

"Three days to my mind was a risk.

"It would have been the same for Ferdinand.

"If he had played on the Sunday I would have had my doubts whether to play him on Wednesday."