Ferguson explained that Ferdinand's injury should not prove too serious, after he was substituted despite the match being precariously balanced towards the end.
"Rio got a bit of stiffness," said Sir Alex. "He gets that, you know, from time to time, the stiffness in his back. So we brought on Chris Smalling as a safeguard.
"If Rio had played it out, he would probably have managed it okay but then not be available for a couple of weeks so I think we've done the right thing at the right time."