Boss Jim Magilton was disappointed with a poor display from his side in what he called an “awful game”. The Town manager felt his players switched off for the Tykes' injury time winner.
Magilton said: "It was an awful game. There was no football played, and I thought we were poor.
"We didn't pass the ball at all well and it was a case of 'after the Lord Mayor's show' for us after last week.
"But neither goalkeeper had to make a save, and we fell asleep when it really mattered yet again.
"Barnsley are fighting for their lives and have just lost a manager who their players had a lot of respect for.
"We expected a backlash - and we got it. Ultimately, we lost the game because we switched off.
"It was a real sickener to concede a goal right at the end and it was a poor performance from us.
"We have to learn the lessons but having admitted we were nowhere near the levels of last week, we should never have lost the game either.
"At the moment we are too inconsistent. We are switching off in matches and that is unacceptable.
"It's something we have to put right and quickly because we have to find a level of consistency in our performance if we are to do anything in this division this season."
Barnsley caretaker-boss Simon Davey was pleased with the way a difficult week had ended: "It's always hard when a manager leaves, and everything seems to be in turmoil.
"I came in on Tuesday and had to prepare the team and made sure we got a performance today.
"We've done that. I think debuts go one way or the other, and Grant has scored - so I'm pleased for all four of them.
"It was always going to be a hard game, and we were under no illusions about the task facing us - given our position in the league.
"I said before the game that if we win and we win 'ugly', then we'll take that.
"I'd rather do that than pass a team off the park and lose. It was a sweet pearl to get the goal two minutes into injury time, and I'm proud of them.
"I thought that if somebody could get the chance we could finish it - because the finishing has been sharp in training this week."