There will only ever be one SAF. No one (based on current approaches) will get close.
JM has never been at a club long enough to do the one thing that SAF stands above everyone on - building and rebuilding squads while still getting a high level of performance out of them.
SAF always had a balanced squad in terms of age and experience. Eg, when Rio and Vidic were past 30, he bought in Smalling and Jones for his next squad.
I am going to give JM some credit for playing some youth players this season, which has been a change from the past.
It will be very interesting to see who he signs in the summer - what age and experience they are, and who leaves.
Carrick, Zlatan and Rooney are the older players, so ones that are 20-23 yo to replace these guys are key. They may not be ready now (unless one is Greizmann, then i think he is good enough haha). Young seems like he will leave too, so another 4 signings in the summer of a fwd / winger, another fwd, a defensive mid, and a fullback - and JM will have a great squad and will be close to rebuilding at the same time.
SAF was very lucky to ride the wave of his era. There is unlikely to be another, because the landscape of the game has changed to so utterly. Another manager won't get the chance to have such a long period without winning something. Also with the amount of money in the game winning championships and cups doesn't really give you any material advantage.