Well after the jesus injury, I do agree that it would be a lot harder for Arsenal to win the tournament as well. I am not saying that it would be impossible, but it would be a bit harder than usual and it would be a bit more difficult.
I do agree that they do have a big chance, between Juventus, United, Barcelona and Arsenal, these four teams are the big clubs that could have a chance at the title but aside from them I am not sure if there are any teams that could win the title and it could be quite difficult to achieve. Long story short Arsenal is not the biggest candidate to win, whoever wins out of United vs Barcelona will be the biggest chance to win, but Arsenal is not too far from it neither.
I agree with you that one of these four teams for sure will be the winner of the tournament.
But I think as you said, the winner of Manchester United versus Barcelona might also win the tournament as well. Gabriel Jesus getting injured has been a big blow for Arsenal. I personally don't think Juventus have been as good as they used to be 2 or 3 years back.
So, I guess one of Barcelona or Manchester United might have an edge over the other two teams. But I think Barcelona have the most chance as United have been really inconsistent in terms of their game. On top of that they have even released Cristiano Ronaldo which is quite unfortunate. I think only time will tell who the champion will be.