Man City are heavily dependent on Aguero, Silva, Hart, and (especially) Kompany - and all of them have been injured at some point recently. Toure has been sulky and less than convincing of late as well. They are still the favorites, but just barely.
Man Utd have a few significant defensive injuries and Martial has gone off the boil a bit, while Rooney and Depay continue to be off-form. They continue to be very hard to beat, but are also drawing too many matches. Every successive draw continues to erode the supporters' tolerance for LVG's playing style.
Chelsea are surely out of the top four picture now, and for all their heroics I can't see Leicester staying up there come May. Liverpool and Spurs are in good form at the moment but have lots of work to do to make a convincing case for a top four finish.
So despite the injuries, I don't see Arsenal's position as being as terrible as some say. None of their rivals are head and shoulders better off than they are.
Each team's fortunes in Europe will be a factor as well. If Arsenal drop out of Europe entirely, that could be worth a place in the league table...