Ronaldo spent seven years wearing Sporting green before he left for Old Trafford in 2003 with a £12.2million price tag and became a megastar.
But the brilliant Portugal winger had warned there would be no old pals act at the Jose Alvalade Stadium, the ground on which he first came to Sir Alex Ferguson's attention in a friendly meeting between the teams four years ago.
And Ronaldo matched his words with deeds as he arrived with perfect timing to finish off Wes Brown's low cross and provide the Red Devils with only their third Champions League away win in 15 encounters.
Having refused to state yesterday whether he would celebrate any goal, Ronaldo restricted himself to a low, double-fisted salute before an almost apologetic bow to the home supporters, who offered the winger a standing ovation when he was replaced by Carlos Tevez four minutes from time.
His efforts ensured United headed home with three precious points - but Edwin van der Sar had equal claim on hero status among the visiting faithful, the veteran Dutchman producing two outstanding saves to deny Liedson and Tonel.