On a performance basis over 2017 I'd actually put Umtiti in the top 3. Guy is an absolute monster and he's proven it in almost every single big game he's played for Barca. Think of any of the best defenders of this era or past ones and Umtiti has been playing at that level pretty regularly.
If he performs at exactly his current level consistently over the rest of his career and wins a handful of big trophies, we're talking about an all time great CB. If he improves at a steady rate, as is expected of his young age, and gets to an even higher level and performs there for the rest of his career, then we are talking about an all time great CB.
All in all, the point I'm trying to make is that it's very plausible he has the ability to be a historically talented defender. There's no one from his generation that's on his level, maybe not even on his tier. If we look back to CB's of the past 10 years or so only a handful have performed at the season-long levels that Umtiti has produced in his 1.5 seasons so far and if he continues to be this good then even fewer CB's have managed to do so over a long period of time.
Someone like Ramos is gonna be an all time CB legend because of clutch goalscoring ability and scattered/inconsistent albeit potentially high level defense. Others like Thiago Silva, Boateng, etc. will also be seen as among the best of their generation despite having poor longevity and not consistently playing at their best. If Umtiti can keep this up for a period of years, he certainly deserves to be recognized among this group of players and maybe even better.