Ferguson feels his threat on the flank is more vital than his menace in the middle.
Wayne Rooney and Carlos Tevez have formed a deadly partnership up front in the absence of injured striker Louis Saha - while wing wizard Ronaldo terrifies full- backs, lays on crosses and has still scored 27 times.
Boss Ferguson said: "There have been occasions, especially when Saha was out and we were relying on Rooney and Tevez, where we've considered playing Ronaldo through the middle. He did it against Birmingham.
"He would be a great threat but if you take him away from the wing you lack penetration."