Steve McClaren warned after QPR's resounding 4-1 defeat at home to Preston that a repeat performance next weekend against League One Portsmouth would see them exit the FA Cup.
The R's had come into the fixture off the back of a first defeat in six Championship games against promotion-chasing Sheffield United.
But the west Londoners were distinctly second best for most of the contest as Jayden Stockley, Jordan Storey and Brad Potts all opened their account for the visitors.
Alan Browne also netted for North End and, although Matt Smith netted a late consolation, McClaren had no qualms about the result.
"You always get bad days at the office and that was a bad day at the office," he said. "That wasn't like us. It's the first time in a long while I've been disappointed with the performance.
"It's happened before and will probably happen again but the best thing you can do is come in on Monday morning and get the work done on the training field. If we perform like we did today against Portsmouth it will be a struggle.
"I can't fault the players' attitude. We're making no excuses about today. Everyone has to be 100 per cent and we weren't."
Preston's opener came courtesy of a mistake in the hosts' rearguard, as goalkeeper Joe Lumley's short pass to Josh Scowen was easily dispossessed by Browne, with Stockley brilliantly curling home the loose ball into the top corner.
"That's the first time for quite a long time I've been disappointed to concede a goal like that," McClaren said.
"It happens and we want to try and build up from the back, but you've got to recover from that and we didn't really.
"We had to change it at half-time [with Smith's introduction] and go more direct. It was nearly effective in the first 10 minutes of the second half and if we'd scored then it might have been different, but overall Preston deserved to win."
McClaren's counterpart Alex Neil was delighted with the contributions of several key players, with Sean Maguire making his first start for more than two months.
"We have spoken so many times this season about our injuries and quality players missing, and although we haven't got them all back you saw Ben Davies, Ben Pearson and Sean Maguire there," the Scot said.
"We are certainly a different proposition with those in the side and fully deserved to win."
Stockley only signed in January from Exeter having scored 15 goals in 24 league games for the Grecians this season and, despite the mix-up, he finished with aplomb.
"There were very good finishes and nice to have the three boys getting their first goals," Neil said.
"Jayden is 6ft 3ins and a target man, but you saw he is decent with his feet too. He has still got work to do on finishing, to be as good as he can be."
