The Reds have conceded 10 goals in the seven games Ferdinand has sat out this season and the manager hinted on Tuesday that the England captain could be in line to make a return against Spain's current league leaders.
But Sir Alex said he won't make up his mind whether to start Rio until the day of the game.
"It's a decision I'll take tomorrow," he told journalists on Tuesday evening at the Mestalla. "He's had plenty of training and a couple of Reserves games - he also played against Rangers and Scunthorpe - but he’s been out for five months.
"That’s why I didn’t risk him on Sunday against Bolton. I felt that would be a physical challenge so we decided to stick with Nemanja Vidic and Jonny Evans, who have done very well for us. I have no complaints about their performance levels."
Fitness clouds also hang over Ryan Giggs, Paul Scholes and Wayne Rooney, although none of those players travelled to Spain on Tuesday morning.
Rooney, who picked up an ankle knock at the Reebok Stadium on Sunday, is looking at "two to three weeks" on the sidelines, while Paul Scholes faces a 10-day lay-off with a calf injury.
"You don’t want to lose your best players, that goes without saying," the manager said. "It was a big decision to leave Scholes behind. He's been in terrific form and now we'll have to adjust the team."