Well Zlatan scored 28 goals last season, despite missing a host of chances and missing the last 1.5 months of the season. Ronaldo is younger and sharper than Zlatan and could conceivably score 35 goals for atleast the next 2-3 seasons, that is to say until he's around Zlatan's age.He would need to average 25 goals for 7 seasons. The chances of that happening are extremely unlikely especially considering he's going to less effective with every passing year, and also that it's much easier to score goals in a Real Madrid side and La Liga, than this current United side in EPL.
After that even if he declines he could score the remaining goals over 2 seasons. If he comes and stays here 5 years, he will break the record I think. And that is truly one record that will never be broken. There may always come a guy who might win 4-5 Ballon D'Ors , but someone who ends up being the record goalscorer of the 2 biggest clubs in the world?? I doubt it could ever be replicated. Hell how many people are record scorers at 2 different clubs, never mind two clubs the stature of Real Madrid and Man Utd?