I think it's too early to write off any team currently in the top four. As a United fan I am obviously very happy with the start we've made, but I'm not getting too carried away. I'd personally still give City the edge in the title race. I think they've got the most balanced squad in the prem, plus they aren't overly reliant on any one player, they can afford to lose Aguero to a rib injury and still score plenty of goals, they could lose David Silva or De Bruyne and still be fine in midfield. Arguably the only worry for them would be if Jesus also now got injured or if they lost another fullback, what with Mendy out long-term.
Chelsea also have a decent squad, but I think they have a few too many new faces in the squad and are a bit weak up front, which has come back to haunt them with Morata getting injured.
Spurs are playing well at the moment, they have the most in-form striker in the league and their starting XI is excellent. They've also got decent cover now at centre back and full back, both positions they were a bit lacking in depth last year. Their biggest worry would be an injury to Kane, plus they would definitely miss Eriksen if he were to get injured.