City is just one club you should be worried about, though. Chelsea are apparently going mad this window, and even though Spurs and Arsenal aren't considered title challengers, with their improvements, that will make 4 teams that United won't be guaranteed victories against, plus Liverpool away. City, meanwhile, will still have a stronger squad overall and will be beating all those teams.
Chelsea, I could see as a challenge, but I'm really not sure how good of a manager Di Matteo actually is.
Spurs, Arsenal and Liverpool aren't a worry at all. United did better than City against the "other" teams this year and the league was decided by the 6 points they took off us. Those are the games that will probably decide it, again.