Chelsea made the worst possible start to their title defence on Saturday afternoon, with the Blues losing 3-2 at home to a Burnley team that only won one away match in the Premier League last season.
Merson has said that Chelsea, who had Gary Cahill and Cesc Fabregas sent off at Stamford Bridge, "have to go out and buy" if they are to be competitive in the Premier League and Champions League this term.
"You look at Man City's bench compared to Chelsea's and it is chalk and cheese. This is not a bad team overnight and next week they go to Spurs at Wembley with no Cahill or Fabregas," Merson told Sky Sports News.
"As a Burnley player, you see their bench before kickoff and there is only Morata on it. The next couple of weeks are massive for Chelsea football club. With how strong this Premier League is, you cannot afford to drop points at home to Burnley.
"You just don't want to see what happened the year before last at Chelsea, when they just fizzled out and didn't make a challenge. They have to go out and buy. No disrespect to the kids on the bench, but if you want to win the Premier League and have a go in the Champions League, you have to have more."