Ronaldo, who joined United in a £12.24million deal from Sporting four years ago, capped his current side's best move of the night as he headed home Wes Brown's 62nd-minute cross from close range.
But United were indebted to Edwin Van der Sar and two excellent saves from the veteran goalkeeper after failing to find their fluent best on a frustrating night at the Estadio Jose Alvalade.
All the pre-match talk was of Wayne Rooney's return to the starting line-up after five-and-a-half weeks out with his latest metatarsal injury.
But the England striker was often isolated in attack as United toiled in the opening 45 minutes.
Indeed the home side wasted two half-chances in the opening 10 minutes when Leandro Romagnoli's deflected shot dropped just wide before Marat Izmailov snatched at a close-range shot that flew well over.
But Van der Sar was to deny them their best opportunity of the match in the 28th minute when he produced a stunning save to finger-tip a rising Liedson drive around his upright.
The Sporting striker couldn't believe his luck as he had picked his spot from the edge of the box and executed his shot with near-perfect precision.
After the interval United began to assert themselves in the midfield area and Ronaldo became less of a peripheral figure on the right wing.
He came close to teeing up Ryan Giggs with a tantalising cross on the hour-mark before breaking the deadlock himself two minutes later.
United's first real move of quality led to Brown delivering a pinpoint cross from the right and Ronaldo dived in front of his marker to divert the ball into the back of the net from close range.
The former Sporting favourite declined to celebrate his goal in front of his former fans but almost tortured the home defence with a couple of mesmerising runs before being replaced in the final 10 minutes.
But before then Van der Sar had produced another top-class save to ensure United returned to Manchester with three precious Group F points.
This time he palmed away an Antonio Tonel header from just eight yards and got back up just in time to see Yannick Djalo lash the difficult rebound wide of the target to the relief of his static defence.