I haven't watch Grujic at Hertha so maybe he's improved a lot but for me he's an 8. In no way do you want him playing a 6 in our system, his passing is no where near good enough.
According to whoscored - admittedly not the best source - he's got a lower passing success rate than Fabinho (81% to 85%) but also scores more highly for key passes per game and scores more - which suggests that he's either playing the role more riskily or not playing as a lone DM. But certainly from what I've seen and heard of him he has no technical weaknesses there, passing is one of his strengths.
It also looks like he's making a lot of tackles and interceptions alongside that good offensive play, as well as being aerially dominant.
That's what I find intriguing about him - no, he's not the finished article yet but in terms of his attribute set he's an incredibly complete player. The only thing he doesn't really have is pace, but then neither does Fabinho. He's tall, dangerous in the air, an accomplished passer and seems increasingly able to use his physicality to win the ball. On top of that, he's got a very good shot - probably better than anyone else in our midfield at the moment. So physically and perhaps technically he's got the attributes to be able to dominate his zone, the kind of player who can, if he's able to use those attributes and if he has the tactical ability, give us a solid platform in both directions.
Moreover, he's young enough and coming from a situation where he'd likely to be happy with a solid squad role which, frankly, is all he's going to get right now with Fabinho as our 6, and with Henderson a) likely to lose his physical gifts in the coming years b) having played very well in a more advanced role c) likely to be needed at RB cover and d) needing very careful management of his minutes anyway then it might just be the perfect time for Grujic to get a chance to see what he can do here.
That said, he had a horrible injury this year and is also an absolute mainstay of that Hertha side, there's not necessarily an urgent rush to either bring him back or sell him when he's actually developing very well in a stable environment in exactly the position we'd want him to learn better.