The problem forced the England centre back to sit out Sunday's dramatic 3-2 home win over Aston Villa in the Premier League, a match where United's defensive unit looked unusually vulnerable.
With Ferguson wary of the threat posed by Porto and their powerful Brazilian striker Hulk, he admitted there needed to be a big defensive improvement if his side are to take a commanding advantage into the return leg in Portugal.
"We knew it was a difficult situation (against Villa) but we chose the right options in terms of the fact players were coming back from international duty," Ferguson told a news conference.
"Most of the players will be okay and we can bring some fresh faces in but Rio, no, I don't think so.
"For most of the game against Villa we looked fragile defensively. There was that fragility at the back but we can change things. The win in the end has galvanised the whole club."
The holders will be boosted by the return from domestic suspension of Nemanja Vidic, Paul Scholes and Wayne Rooney, who all missed the Villa game which was decided by teenage substitute Federico Macheda's stoppage-time winner.
The 17-year-old's reward for his dream debut was a place among the substitutes for Tuesday's match at Old Trafford.
"The young boy came on and did an incredible job. It was just one of those incredible moments," said Ferguson.
"I think he will be involved off the bench. We've got Wayne Rooney back, which is a big bonus, but it's difficult to leave him (Macheda) out."
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