Mark McGhee described his Motherwell side's performance in the 2-0 loss to Kilmarnock as "diabolical".
The manager was stunned by his team's showing as defeat took Well into second bottom spot in the Premiership.
The Steelmen have gone six league games without a win, while Killie have moved out of the relegation zone.
"I'm really disappointed it didn't look anything like I imagined it was going to look after the way we prepared all week," said McGhee.
"I thought we prepared right, we had a way of playing in the first half which just didn't materialise at all; we really log-jammed, we had nowhere to go, we had nowhere to pass the ball.
"We changed it at half-time and at that point I thought we might just up our game little bit and go on and win the game because I knew we had so much more to come.
"It never materialised and we ended up losing the game deservedly.
"It's the poorest performance since I've been here and now we have to go away and work out what's caused us to be as poor as we were today.
"The performance was diabolical."
McGhee fears the result will dent the confidence of his players as they prepare to face bottom side Dundee United on Tuesday evening.
But he admits he knew the size of the task he faced when he landed the job for a second time in October.
"I would remind people I was brought here not because we were flying, but because work was needing doing and we are in that process," he explained.
"Occasionally we are going to have setbacks like that and setbacks like the run we've had recently but we have to as quickly as possible to find a way out of it.
"So it's up to me and the coaches and the players to dig ourselves out and prepare for Dundee United and go and have another go."
Kilmarnock caretaker boss Lee McCulloch was the opposite of his Motherwell counterpart as he assessed the impressive win courtesy of outstanding second-half goals from Greg Killie and Craig Slater.
"I'm obviously delighted with the performance, but I think the most important thing today was getting the points," said McCulloch.
"I was especially pleased in the second-half performance. We created chances and the Motherwell goalie's made a good few saves and we maybe could have had one or two more goals today.
"While I'm delighted with the performance it's only the start of the hard work.
"It's a pleasing day but we are in again tomorrow, we've had no days off for about two weeks, but we have a big game on Tuesday and it's just all about working hard and keeping everybody together."
As Killie still ponder the options for their managerial vacancy, McCulloch is likely to be in charge for the midweek Scottish Cup replay with Rangers, the club he left in the summer.
Were he to guide Killie to the last eight of the cup, the Rugby Park directors may well have to seriously consider McCulloch taking charge on a formal basis.
"Nothing's change for me it's just a day at a time," said the 37-year-old. "I don't know what will happen from now on, but I'm delighted with the players today."