Scott Parker admitted he was shocked by his Fulham side's 3-0 home defeat to bottom club Barnsley and apologised to the club's supporters for the manner of the loss.
The Cottagers lost ground on second-placed Leeds with a three-point gap now separating Parker's side from the automatic promotion places.
And Parker admitted he found it hard to explain his side's display after Barnsley secured victory through two goals from former Fulham striker Cauley Woodrow either side of a Jacob Brown strike.
The Fulham manager said: "I am shocked, I am bitterly disappointed, I am gutted. I didn't see it coming. The fans who came here didn't see it coming and I can only apologise for the performance today because it wasn't what we have come to expect.
"But these things can sometimes happen and we need to reboot and learn from them. And the most important thing is to keep going, keep being strong characters in the changing room and don't dwell."
He added: "There are no excuses. We were beaten by a better side today, a side that had more intensity and we were second best really. We didn't deserve anything out of the game.
"This is a bump in the road. This is what this league brings, there are no surprises. When you are playing a bottom of the league team who come here and are fighting for their lives, they can do this to you and we need to learn from that.
"You are dealing with human beings and that can happen. Did I expect it? No. But it's something we need to learn from, reboot and go again."
Fulham keeper Marek Rodak fouled Brown to concede the 25th-minute penalty converted by Woodrow, and was at fault when Brown chipped in the 51st minute.
Parker said: "He's made two errors today and he will be the first to hold his hands up and say that. But he has been nothing short of fantastic since he came in the team.
"The biggest challenge for Marek and for all the players is during these moments let it kill you, let it burn inside.
"But the true test of great teams, the true test of character of great players and great people are that when you come back in on Monday morning you have to be brighter than you've ever been."
Barnsley manager Gerhard Struber challenged his team to maintain the standards they set at Craven Cottage after closing the gap to safety to six points.
He said: "I think the last few days was a big challenge for us, especially being at the bottom of the table and the last game was not a big highlight for my team.
"It was a very, very important win for my team and gives my team and staff a good mood to fight in the next few days and prepare the team for the next challenge against Middlesbrough.
"The gap is six points now and I think if we can reproduce this style in the next few weeks it won't be easy for any team we play.
"We don't look at the table. The only thing we can influence is our performance in the next game and we need the right atmosphere."
And Struber singled Woodrow out for praise. The Austrian said: "He is a very important player for me and for the whole team.
"He is a leader in my team, today he created a very good game and he worked very hard off the ball. When Cauley is in this mood he is a big player. He showed me that today."