Chelsea were held to a goalless draw by Arsenal last weekend, dropping points for the first time since their opening day defeat to Burnley.
But Merson is confident Chelsea will earn three points when they travel to the bet365 Stadium this weekend.
The Sky Sports pundit has been far from impressed by Stoke so far this season, with the Potters taking on the Premier League champions on the back of a 2-0 Carabao Cup defeat to Championship side Bristol City.
Merson told Sky Sports: “Chelsea were poor against Arsenal – bar the Pedro chance they didn’t create much – and it just highlighted how much they miss a fully-fit Eden Hazard.
“This is a match they have to win because they cannot give the two Manchester teams any more of a head-start.
“If they lose at Stoke and City win, that’s a six-point gap in September.
“I have been disappointed in Stoke this season, the Newcastle game was a bad result. But what really gets me is they lost 2-0 to Bristol City in the Carabao Cup. That’s a joke.
“Stoke aren’t going to challenge for the league and they aren’t getting relegated.
“It’s another mid-table season. So why not go for it in the cup? It was a shocker.”