I think he will be, but you have obviously seen more of him than me. Can you elaborate a bit more?
He is a flair player, which is great for any team and I think especially United could use one right now. He is flashy, has a lot of tricks up his sleeve, which makes him great to watch in the highlights. He is a great dribbler and has decent amount of accelaration and top speed. However, I doubt whether his dribbling and flashyness translates to the prem, as he will get a lot less time and will be faced with stronger opponents. That combined with his relatively weak physique and low shooting power makes me doubt whether he is up for it, al least right now.
I do see other Ajax posters being more positive about his chances in the prem, so it could very well be me that is misjudging the situation.
EDIT: I think what also plays a role in my estimation is that I've been too positive about attacking players making the step up from the Eredivisie to the prem in recent years. I really did think that Bergwijn, Ziyech and Donny would be great here, but it hasn't worked out for them. Maybe that's why I'm more reluctant in expecting Antony to succeed. Mind you, especially Bergwijn and Ziyech were very impressive in the Eredivisie, a lot more so than Antony is in my opinion.