On Tuesday evening the Portuguese attacker will run out on the Old Trafford turf for the first time since making the £80 million switch to the Santiago Bernabeu back in 2009.
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And, when asked whether the 28-year-old has since established himself as the best player in the world, Sir Alex told ITV: "At this moment in time I think he is."
"Ronaldo's a special player, we know all about that.
"He's a mature man now. I think he'll get a great reception, but he's got a job to do - he plays for Real Madrid now - but we'll do our best."
The Scot also agreed with Jose Mourinho's declaration that the visitors will score at Old Trafford, adding: "I think they will. But so will we."