The single factor making Manchester United the favorite to win the league this year is their defense. Manchester United the league's best team defensively. I'm at a loss for words when it comes to describing Martinez. His addition to the team has boosted confidence. He and Varane make an amazing team. Manchester United's defensive line has been lacking this for a long time. Another Vidic, this one diminutive but powerful. I'm thrilled for him and I hope he keeps performing well.
I don't really agree with saying Manchester United is one of the best defensively because looking at them now they are actually weak in their defence.
Seeing how many goals they have conceded is definitely proof because they have even conceded 16 goals from 12 matches and if you look at the goal ratio it is still better Brighton, West ham and even Newcastle United so there is no clear reason why you say they are good enough on the defensive line.
Yes, even Newcastle look better with fewer goals conceded but Newcastle are also very good in scoring so far, as Newcastle have 24 goals and conceded only 10. As for Manchester United they only have 17 goals and 16 conceded, it is a bad record even though Manchester United are currently in fifth position in the standings.
Instead of judging the performances of Martinez and Varane who have just made a good impact for Manchester United, I think there will be a better player too, that is Harry Maguire who is still trusted as team captain. It's possible, with Harry Maguire who is still believed to be the captain of the team, it is possible for Manchester United to win the title this season.

I'm just shocked when people say Manchester United are title favourites, because the truth is they're recovering and it's only happening slowly. Even in my view, Newcastle are a better team and deserve to be title favorites this season than Manchester United.
Newcastle's way into the top four in the standings is a great way because they don't have a luxurious squad like the squad that Manchester United has, but Manchester United is still difficult to get good results in every match.