Not sure how you can make that statement of him playing any position. As far as i know (and if im wrong i stand corrected) he's only played in the middle. It doesn't mean he can play any position and be world class. Gerrard has played and excelled at CM, RB, LB, LM, RM and as a number 10. His never say die attitude saved us many times. Im not saying Keita doesn't have world class potential. But there's world class potential and then there's being better than gerrard. Since we've been linked with him i've been watching him a few times and i'm proper excited as he looks an amazing talent who will suit our system well. But better than gerrard? Nah i just don't see it.
He's played #6,#8,#10 and as a winger in their system so far.
When people say that though, they simply mean they have the tactical intelligence to play any role on the pitch. As a #6 for example, especially in the system they play, you end up at fullback a lot of the time in transitions. For this reason most Brazilian fullbacks have play as #6 and vice versa in their youth.
And nobody is saying he is better than Gerrard now, or will be in future but there is an argument he has the potential to hit or pass that level, which is exciting. Doesn't mean he will of course. If you look at pre-Rafa Gerrard for example, he was a good player but I wouldn't say world class. Rafa transformed him and his production sky rocketed once Rafa could add some tactical intelligence to his game on top of the technical and physical gifts he had.
I would argue that 22 Keita looks a better player than 22 Gerrard did for sure. But then that's because Keita's all round game is at a very high level whereas Gerrard hadn't had his tactical game worked on at that point. Therefore at 22 Gerrard had a lot of space to improve his weakest areas of his game.