Chris Wilder admitted the Blades had been beaten by the better team on Tuesday evening.
Speaking after the reversal at Craven Cottage, where two goals from Newcastle loanee Aleksandar Mitrović paved the way for the hosts, Wilder admitted the loss was a harsh learning curve for his players.
He remarked: "It was a tough night. We come up against some top end, experienced Championship players, at a club pushing hard to get out of the division.
"They've got some Premier League players in there too and with that comes the quality, it was a difficult night for us and I felt for the players, whether to press, whether to drop off."
Wilder added: "On night's like this you have to take your chances. We made a fantastic start and should have been in front after 10 minutes, then they produce a bit of quality and we're chasing the game.
"We then have the best chance of the match to make it 1-1, we miss that and in a heartbeat the ball is in the back of our net again following another fantastic bit of movement and a clinical finish from a player that should be playing in the Premier League in my opinion."