The Portuguese joined United from Sporting in 2003 as an 18-year-old and left plenty of great memories before heading for Real Madrid.
Here, the PA news agency picks out five of the best goals Ronaldo scored during his first United spell.
Man Utd 2 Portsmouth 0 – January 2008
Ronaldo developed a 'knuckleball' free-kick technique and rocked Old Trafford with an effort against Portsmouth's England goalkeeper David James at Old Trafford. Positioning the ball with the valve facing him, Ronaldo's aim was to strike it dead centre and slightly underneath with the instep of his right foot. Against Pompey, and 25 yards out, Ronaldo connected with the ball perfectly as it cleared the wall and dipped viciously into the top right corner. It was a technique many tried to copy but few failed to master.
Roma 0 Man Utd 2 – April 2008
Big-game goals and unstoppable headers – two Ronaldo trademarks. United took a giant stride towards the Champions League semi-final as Ronaldo made a very difficult Rome assignment a whole lot easier. Paul Scholes chipped a weighted cross into the box after smart work from Wayne Rooney but there appeared little danger to the home defence. That was until Ronaldo sprinted into the box to produce a phenomenal leap and header which gave Brazilian goalkeeper Doni no hope of stopping.
Man Utd 1 Chelsea 1 (United won 6-5 on pens) – May 2008
Not the most dazzling Ronaldo goal, but it came on the biggest stage of his time at United: the 2008 Champions League final in Moscow. Ronaldo had giving stand-in Chelsea full-back Michael Essien a torrid time before promise was turned into joy after 26 minutes. Wes Brown sent over a cross from the right and Ronaldo soared above the helpless Essien to head United into the lead. It was his 42nd goal of an extraordinary season. Chelsea equalised and Ronaldo missed his penalty in the subsequent shoot-out – but it did not matter as United lifted the trophy.
Porto 0 Man Utd 1 – 15 April 2009
This was probably the best goal Ronaldo scored during his time in England. Porto had held United 2-2 in the first leg of a Champions League quarter-final tie at Old Trafford and Ronaldo returned to Portugal with a point to prove. It came as early as the sixth minute at the Estadio do Dragao when he picked up possession inside the Porto half. No-one could imagine what was to come as Ronaldo launched a 40-yard missile that Porto goalkeeper Helton got nowhere near. It also proved the only goal of the game as United set up a last-four tie with domestic rivals Arsenal.
Arsenal 1 Man Utd 3 – May 2009
After a first leg which United shaded 1-0, the semi-final was in the balance. Not for long, though, as United raced into a 2-0 lead after 11 minutes with Ronaldo producing an extraordinary free-kick from 40 yards. But his second goal probably lives longer in the memories of United fans. Ronaldo started a stunning counter-attack with a deft flick to set away Park Ji-sung, who took the ball in his stride and found Wayne Rooney on the left flank. Ronaldo, sprinting at full speed, raced on to Rooney's pass to smash the ball past Gunners goalkeeper Manuel Almunia and finish the game.