The Portuguese forward stole the show, receiving an immaculate reception from the Old Trafford crowd, with plenty of 'Ronaldo 7' shirts filling the stadium, with his return completing one of the most remarkable fairy tale stories in football history.
While the 36-year-old led his team to victory, there were some cracks in his performance, and as a result, we have highlighted the pros and cons that we think United will be given with his signing.
Pro - Marketing and commercial boost
The most obvious benefit is that Cristiano Ronaldo will generate an incredible amount of shirt sales due to him boosting the club's brand even more.
United are already a huge club globally from a marketing perspective, and signing the five-time Ballon d'Or winner will only increase it even more, as can be seen through the fans' reception on Saturday.
Con - Basing the play style on him
One of the main issues that Juventus had while Ronaldo was at the club was basically that they were revolving their whole style of play around the Portuguese forward, and there were signs that United players were shifting towards this mentality as well on Saturday.
Several players were guilty of looking to play the ball to Ronaldo far too many times against Newcastle, and while he has been signed to be the star forward, other players have to also trust their own abilities in order to make a difference in the final third.
Basically, United cannot depend on hoofing the ball to Ronaldo expecting him to be the difference-maker, or else a proper man-marking job will end up completely cancelling them out.
Pro - Goals.
One thing for sure is that Ronaldo will guarantee United goals.
Throughout his whole career, Ronaldo has shown that he has what it takes to provide a steady output of goals, even during his first spell at Old Trafford, winning the Premier League Golden Boot in 2007/08 with a remarkable tally of 31 league goals in 34 games.
He has got off to a brilliant start with a very good double, finding the space that plenty of other strikers struggle to find, most notably with his first goal, being right on the edge of the six yard box to profit from a fumble from Newcastle goalkeeper Freddie Woodman.
As his numbers show, he will get United goals and provide them with a completely different option to other players up front.
Con - Competition for places
One thing that was severely questioned with the signing of Ronaldo was that other forwards will have their game time immensely limited, especially since United are relatively well stocked in terms of attacking options.
Given Ronaldo's ability, he will definitely start most of United's games, despite his age, and as a result, the steady development that Mason Greenwood has made, along with other notable players that are keen to make their mark, such as Marcus Rashford and Anthony Martial, who will end up playing on the wings rather than up front, could be hindered greatly.
There is also the curious case of Edinson Cavani, the 34-year-old forward that stole the show with his energetic performances last season that got him 17 goals in all competitions, and will now feel that he is definitely second choice with Ronaldo back at United.
It will be interesting to see how Ole Gunnar Solskjaer utilises Ronaldo and manages his game time, but Ronaldo himself will surely want to start every game if possible, and so the question remains whether Solskjaer will be courageous enough to take Ronaldo off or to tell him to sit out of a game when needed to do so.
Pro - Genuine hope
The sheer manner of his return, with him being linked with a move to fierce rivals Manchester City, along with the fact that he got off to such a great start, will surely fill United fans with plenty of hope that they can challenge for silverware this season.
Even at his relatively old age when it comes to football, Ronaldo is still scoring a ridiculous number of goals, and the fact that the Old Trafford faithful have got the player that lit up the stadium with some spectacular moments throughout the years, then they will feel that signing him could be one of the final steps to winning a trophy under Solskjaer.
He will be an inspiration to plenty of players and fans, and the ringing noise of the 'Viva Ronaldo' chant that filled the stadium on Saturday will continue to make everyone believe that United have what it takes this season to succeed.
Whatever happens during his return to the club, signing Ronaldo is definitely an exciting move, that will seek to add to the Premier League's growing pool of brilliant players.