Hi, Serbian guy here.
First of all, Sergej Milinković-Savić absolutely isn't a 10. He is an 8, a box to box midfielder. He can play as a 10, but it isn't his natural position and he is best suited for a formation that has 3 midfielders. As you all have said he isn't a 6 and if you are signing him to be a 6 that would be a huge mistake. He's miles ahead of McTominay and Fred, and although I'm biased, he's better than Bruno and Erikssen(although I'd say Erikssen in his prime was better). He's also able too play a slightly more defensive minded game than Bruno and Erikssen.
In Lazio he played as an 8 last season andn in our national team he usually plays in a midfield 2 alongside a defensive minded midfielder(Lukić from Torino or Maksimović from Getafe), which shows he doesn't have to play as offensively as some of you think. He's incredibly skillful and he likes to show it off(he tries lots of backheel passes,no look passes,nutmegs,etc).He works really hard and tries to help defensively a lot(as you'd expect from a box to box midfielder), he was one of the leading players in Serie A when it comes to covered ground and he was our national team leader in the same category at 2018 World Cup.He is good with both feet and can score goals with both(even from outside the box). He's rarely outmuscled, and he has very good crossing ability. I've seen headers were marked as a major strenght here, but I'd say he's better on the ground than in the air. Sure he's got a decent header but I wouldn't even put him in the top 5 in our national team, whereas I think he's incredible with the ball in his feet. The one thing he lacks is speed. He's not extremely slow, but that is definitely his biggest weakness.
His passing range is insane(probably not on the De Bruyne level or on Pogba's level when he's having a good game), but I'd put him in top 10-15 in the world definitely. At club level he's really consistent with his performances and will always give you at least 7/10 performance(but in most games he'd give an 8 or 9). In the national team he isn't as consistent(probably because he struggles a little bit more in a midfield 2), but nevertheless he regularly comes up with an assist or a goal. That's also one of his strongest attributes, he's a clutch player. When a defence needs to be unlocked he always comes up with something.
If he were to come to Manchester United, I think he'd need a 6 behind him. Him, a new DM and one of Erikssen/Bruno would be an excellent midfield. I don't really get some criticism on here, that he's not a good player when he's been one of the best(many fans of Italian football would say the best) midfielders in Serie A in last 5 years or so. Sure, he's not a 6 which is a position where you need reinforcment the most, but to say he's an average footballer is simply wrong. Also you definitely need an upgrade on McTominay as well and he's 100x the player McTominay is.
Good luck with the rest of season whatever happens, but I hope that you'll sign him and that you'll have another Serbian player here for me to cheer on(I miss the Vidić days
)