The United boss has said that Ronaldo, who is yet to play this season having undergone surgery on an ankle injury during the summer, will play some part in the match against the Spaniards at Old Trafford on Wednesday evening.
"Cristiano will certainly be involved," Fergie told press reporters on Tuesday.
"Whether we start him or not, irrespective of other players' injuries, is still a major decision.
"I think it's more likely that he will start on the bench."