And the report claims the Old Trafford club would be willing to match Real Madrid's £80m asking price for the forward should they miss out on Barcelona midfielder Cesc Fabregas.
David Moyes is conscious of reassuring the club's fans as rivals splash the cash elsewhere this summer. United have already lost out to Bayern Munich in the race for Thiago Alcantara.
And should a move for Fabregas fall through, Moyes and United executive Ed Woodward want a contingency plan in place to ensure they still have a chance of landing a top target.
Their likely Champions League rivals next season have all been busy - PSG have signed Edinson Cavani from Napoli, Real Madrid have brought in a couple of young Spanish starlets, Barcelona have signed Neymar and Manchester City have spent more than £90m already.
Even Arsenal look ready to spend big, having had two bids rejected for Liverpool striker Luis Suarez.
The report claims United will wait and see if Real Madrid move for Gareth Bale - if the Spanish giants are successful Moyes will move in for Ronaldo.
They would face competition from Paris St Germain for the 28-year-old Portugal international, who has two years left on his Real Madrid contract.
But United feel they hold the advantage of appealing to Ronaldo's connection with Old Trafford - he won the 2008 Ballon d'Or at the club and was hugely successful during his spell under Sir Alex Ferguson.
And any disappointment at having missed out on Thiago and Fabregas would almost certainly be forgotten if Moyes could pull off a deal for Ronaldo.
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