Chelsea are already four points clear of United and Giggs admits they cannot afford to let them extend that further.
He said: "That's probably been the case over the last six or seven years, so we're wary of that."
He added: "I honestly feel we just haven't had the breaks.
"The concentration lapses are a concern, but we're not too far away.
"If we finish our chances off, we will be beating teams four or five nil."