Sir Alex: Barcelona slump Man Utd wake-up call
United head to Newcastle in a seemingly impregnable position at the Premier League summit.
"Barcelona have been playing Saturday-Wednesday-Saturday since January," said Ferguson.
"They did have an 11-point lead, now it is four and for the first time this season they lost a game after being in front.
"It is a warning, and there are plenty of them, that if you take your foot off the pedal you can get a kick in the teeth."