Speaking after the 5-1 defeat, Flitcroft said, "First half - I was okay with the performance.
"We defended really poor all over the pitch.
"I'm really disappointed with the aspect of how we defended as a team.
"We thought we could snatch a goal and get the game back to two-all. But I'm disappointed with how the lads reacted to goals.
"First half we conceded one goal and I was okay with it. Second half they opened us up too easily.
"It's sink or swim time. Tomorrow morning I better have a group of players who better fight."
Mike Pollitt made his Barnsley debut having only just signed a 28-day loan earlier today.
"I've felt for quite a while Luke Steele was not under enough competition," he added. "I felt I could relieve a bit of pressure on Luke.
"Luke's been rested tonight, and it was a good opportunity to try something different."