Posh, who host Town, were beaten 5-1 away by promoted Bournemouth.
But Town manager Clark said: "Peterborough are one of the favourites to be there or thereabouts at the end of the season and rightly so.
"I don't think it has helped our cause that they are coming off the back of a very heavy defeat.
"Their manager will be smarting and I expect them to come at us from the first whistle.
"Before that result it was always going to be a hard game anyway. But that will probably make it even harder."
Before crashing on the south coast, Peterborough had started impressively with a 3-0 win over Bristol Rovers being followed by a comprehensive 4-1 defeat of Rotherham in the Carling Cup.
Town, with four points from two games, travel to Everton in the Carling Cup on Wednesday.