The Czech international believes the Blues have dropped too many points against inferior opposition this season, and he has urged the side to find a way to win ugly.
Rafa Benitez's side has won 19 league games this season compared to United's 27, and sits 20 points behind the Red Devils in the league table, battling to finish in the top four. Cech has blamed Chelsea's failure to challenge for the title on an inability to grind out wins, which has resulted in dropping points against lesser sides.
"This is where we count the losses of the season," Cech told The Independent. "This is where it cost us the points to challenge the Manchester teams.
"To win the title you have to be consistent, and [United] have been consistent. You don't win the title just playing nice football.
"There were times this season when Man United didn't have a particularly great game but they knew how to kill the game off and score at the right time.
"They knew how to defend it, that was the difference. Man City had the same season as us, dropping points at places nobody expected them to."
Chelsea is now locked in a three-way battle with Arsenal and Tottenham to qualify for the Champions League, and Cech believes Wednesday's match against Spurs could decide that battle.
"We know we are capable of playing well and beating anyone, but the frustrating part was that every time we seemed to get back in the title race we dropped points again," Cech said. "The Tottenham game might be the one, because it is head to head. We will see what circumstances we go into that game with. But the team that wins the derby will be in a good position to get the Champions' League spot."