The Scot insists that the centre-back, who missed a significant portion of last season owing to injury, still needs "another game or two" to regain full fitness.
The defender has missed 61 of the club's last 103 games, while partnering fellow centre-back Nemanja Vidic only nine times in the last 50 matches. In the meantime, questions have been asked of United's back line as they conceded nine goals in their last five games in all competitions.
"Rio has had plenty of training and a couple of reserve games," Ferguson said, according to The Mirror.
"He had a game against Rangers and I think he probably needs another game or two. That goes without saying, considering he's been out for five months.
"That's why we didn't risk him against Bolton on Sunday. We felt it was going to be a physical game there, so we decided to stay with Vidic and Jonny Evans. They've done very well, I don't have complaints. And whether Rio plays is a decision I will take tomorrow. The important thing is that he's coming back and we can look forward to having him back."
Having drawn the first game in the group stage - a 0-0 stalemate with Rangers - the Red Devils boss is under no illusion as to the test that awaits his charges in Spain.
"It was a disappointing result for us in the first game," said the United boss. "But we have five games left to qualify.
"We expect a very difficult game. Valencia are top of the league, I watched them last Wednesday, and on TV on Sunday, and they're playing very well.
"Spain is obviously the hardest place for us to go. Spain has produced cup-winning teams for years and years, you have to recognise that. The standard of Spanish football always been very good, so it will be a tough game. We're all aware of that."
He added: "The Champions League is the best tournament. It's the biggest challenge, because you're playing the best teams. You want to be classed alongside the likes of Barcelona, the great teams who've won it so many times.
"There's a big expectation for us to do well in Europe. Sometimes we've disappointed and sometimes you can be unlucky - one bad moment in football and you can go out. But it drives you on and hopefully this year we can so better."
Ferguson is unable to choose from a full strength squad, as striker Wayne Rooney could be out for up to three weeks owing to an ankle injury, while experienced midfielders Ryan Giggs and Paul Scholes are nursing hamstring and calf injuries respectively. And all three players were left at home to lick their wounds.
Ferguson said: "You don't want to lose your best players, that goes without saying.
"It was a big decision to leave Scholes behind, but with Giggs and Rooney there was absolutely no choice. We're going to have to adjust the team."