The Red Devils have lost three and won two of their opening five games and sit nine points behind leaders Manchester City already.
The likes of Arsenal, Liverpool and Tottenham Hotspurs have enjoyed strong starts to the season, with all three sides unbeaten and sitting two points behind Pep Guardiola's side.
It might be early days yet but Merson believes Man City remain favourites to win the league, and Erik Ten Hag's team can kiss goodbye to the title.
Speaking in his Sky Sports column, Merson said: "Liverpool and Arsenal are the only contenders for Manchester City this season.
"Liverpool were poor against Wolves first half, really poor. But they came out in the second half and blew Wolves away. The problem is you can't be doing that against the better teams. You don't want to be going to the Emirates or the Etihad and playing like that, the game would be all over in the first half.
"You've just got to stick with Man City for as long as you can. Every game that goes by, you have to make sure you're three points away from them, tops.
"For example, Manchester United are already nine points behind. They're finished. Done. They can't win the league - because it's Man City. If it's any other team you can go, 'oh if you put a run together, you never know.' But City are not losing four or five games.
"You have to start well. Look at Arsenal last season. Look at when Jose Mourinho won the league at Chelsea. He always started well, always.
"The title race used to start in December and January - and it was about who had the best run after that. Now Man City have blown it out of the water. You can't fall behind.
"Arsenal haven't really got going and they've won four and drawn one. Everybody is like that, even Man City are not perfect. Fulham had chances against them, West Ham had them on the rack the other day. They were 1-0 up and Ederson was there making world-class saves."