Warne was left seething after the misfiring Millers squandered a welcome 2-1 lead in a disappointing 4-2 loss to the Cod Army.
Michael Smith’s welcome headed equaliser seven minutes before half-time cancelled out Callum Morton’s 24th-minute opener. And Kieran Sadlier’s cool penalty strike in first-half injury-time, after Chiedozie Ogbene had been fouled by Callum Camps.
But Ged Garner rifled between keeper Viktor Johansson’s legs before the hour mark and, shortly after, Danny Andrew curled a fine free-kick into the top right corner.
And, to add insult to injury, midfielder Camps made up for his earlier error by drilling home with 21 minutes left.
“Fundamentally we weren’t at our best for whatever reason,” said Warne. “We didn’t move the ball quick enough and we looked a bit laboured, which isn’t our lads.
“I felt the belief drained out of the lads. The lads have been excellent, but that’s our worst performance of the season. It’s a worry. All that aside I still think we created enough chances to win – I’m not being delusional.
“I’m not saying we played great, because we didn’t, but you can definitely play rubbish and win a game of football. We didn’t play rubbish, but we just weren’t at our scintillating best.
“It’s one of those days when we could have done with knicking a win even though we played bad.”
Rotherham United: Johanson, Ihiekwe, Wood (Edmonds-Green 66), Harding, Lindsay (Grigg 73), Ogbene, Rathbone (Barlaser 66), Wiles, Sadlier, Smith, Ladapo. Unused substitutes: Mattock, Kayode, Odoffin, Vickers.
Fleetwood Town: Cairns, Hill, Clarke, Johnson (Clark 20, Lane 71), Johnson, Biggins, Rossiter, Camps, Andrew, Garner, Morton (Edmonson 84). Unused substitutes: Crellin, Garner, Matete, Johnston.
Referee: Craig Hicks (Surrey).