The R's found themselves 3-0 down at half-time, before Fulham striker Andrew Johnson stole the headlines when he comleted his hat-trick in the second half.
"I think we've had a horrific couple of days really," Warnock told Sky Sports News. "I think if you're ever going to catch us on a bad day, law of averages, it was going to be today.
"It really was Sunday league for the fourth goal. No marking on Andy Johnson and I mean the lad has put his hand up in the dressing room, whose fault it was but it's too late and you can't come back from 4-0 down."
However, the Rangers boss was keen to praise his side's supporters who travelled the short distance to Craven Cottage.
"I thought our crowd was absolutely fantastic today," he added.
"You know 6-0 down, it's difficult for a supporter to get behind a team, but I thought they were outstanding because they know how quickly we've come in such a short space of time and we're going to have bad days in the office."
The defeat has left Rangers in 11th place in the Premier League table, while Fulham have climbed up into 12th.