Fatigue in a ninth match in 28 days and an early Swansea City goal were the key factors in Barnsley's 2-0 loss, said head coach Valerien Ismael.
The hosts took the lead after 75 seconds then Victor Adeboyejo scored an own goal midway through the second half.
The Reds, who had won their previous three matches, have still won eight out of 13 under Ismael and are five points off the play-offs. There were few chances in a game which was played on a very poor pitch.
Ismael said: "It is not possible every time to concede a goal like this after two or three minutes - it is not the first time and it is very difficult to come back every game.
"That was the first thing, the second thing was that Swansea are normally a possession-based team, but they played completely different. It was a fight and the ball was more in the air.
"In the first half, we were in the game and at half-time I told the guys to accept the pitch and go back to basics. There was a lot of fatigue in the second half, the intensity was not there. We took a lot of wrong decisions. We played too early or too late.
"We have to take into account that we performed very very well in the last games and, I have to accept that, such a performance could happen in such a schedule. We have the accept this game, we deserved to lose.
"I don't want to speak too bad. It's my job to see all the situation and criticise if I have to. We have performed well in the last few games and we will in the next games. It is just strange because I want the opponent to be much better than us if we lose – then I can sleep better."
The pitch at the Liberty Stadium was very bare and muddy, and was dug up after the game to lay a new surface. Ismael said: "It was more a problem for Swansea than us. We knew about it before the game but it was really awful to see. You have to accept. We knew it would be more direct but we had to do it with conviction."
The second goal was an own goal by Victor Adeboyejo at the front post defending a corner. He said: "It is part of the game. We have to accept it. It is the second time in a row, first Cauley Woodrow makes an assist against Preston now Victor straight in the goal. We have to work on the positioning with the guys."
The Reds only had eight substitutes. Ismael said: "We had one at home (Callum Styles). I had enough players to change. It was not the problem today."