The 31-year-old midfielder stepped into a defensive role against Barcelona in Gothenburg on Wednesday night in the midst of an injury crisis, and may have to do so again with Jonny Evans, Phil Jones and Chris Smalling all currently unfit.
Ferguson told the club's official website, www.manutd.com: "Smalling is out for three months. Jones got a muscle injury in his back and was sent home from Norway, so the area we're concerned about is centre-back."
He added: "Evans won't be ready for the start of the season. He won't be far away, but he won't be ready.
"Smalling is out and I can't give you an update with Jones because we've not fully diagnosed that.
"But Michael Carrick was absolutely brilliant [in Sweden] with his passing the ball and his interceptions. He was fantastic."