Chelsea moved four points clear of United at the Premier League summit with a 1-0 victory over Bolton at Stamford Bridge last night.
Nicolas Anelka's first-half strike, his first in 14 games, settled the outcome and kept Chelsea on course for an historic League and FA Cup double.
Arsenal have the chance to close the gap against Tottenham in a London derby at White Hart Lane tonight but Cech warned the Gunners and United that one more mistake could be crucial to the destination of the trophy.
"Arsenal have a big game tonight against Spurs because they're the furthest away and if they don't win, the gap will open," declared Cech.
"They know they're under pressure. Then at the weekend there is the Manchester derby and we play a London derby at Tottenham. It can be a decisive week.
"It can change the position in the Premier League a lot if we manage to win our game and the others lose theirs. It could also go the other way and change everything for the last three games.
"But beating Bolton was a massive step forward. We can play good football and score goals but when a game isn't going our way and we have to fight and dig deep, we can do that. We showed it against Bolton.
"I have been at Chelsea for six years and just kept my first clean sheet at home against Bolton so now it's time for our record at White Hart Lane to change too.
"Last season everything went wrong. There was a bomb alert, the game was delayed, we were kept waiting and then we started slow and lost 1-0.
"Even though we had chances, it was not our day. This time it's going to be different for us."