Kilmarnock manager Allan Johnston admits it was a mistake to haul off Alexei Eremenko in the 2-1 defeat by 10-man Inverness Caledonian Thistle.
Billy McKay grabbed the visitors' winner after the influential Finnish midfielder was withdrawn.
"We just felt that, in the last 20 minutes of the game, it looked like he was tiring," explained Johnston.
"But he's got that bit of quality we lacked in the last 20 minutes. So it probably was the wrong decision."
The Kilmarnock fans made their displeasure known vocally from the stands after the withdrawal of the player who gave them the lead from the penalty spot as David Raven was sent off.
"They've got every right," Johnston admitted to BBC Scotland. "That was a poor second-half performance, especially when you go 1-0 up.
"When you go in front, especially in this league when the first goal can be vital, you expect to go on and win the game."
Kilmarnock would have moved above inactive St Johnstone into sixth place in the Scottish Premiership had they won.
"It is a massive opportunity lost," said Johnston. "If we won today, we overtake St Johnstone and we'd be in a very good position to push on for the rest of the campaign.
"The last thing we can do is get too down.
"We've got a small squad and it's being pushed to the limit just now.
"Everyone's competing for places and we've got to make sure we respond in the right manner and win the next few games."
McKay scored both Caley Thistle goals to take his total to four in three games and earn praise from manager John Hughes.
"When you've got Billy McKay on the park, you've always got the chance of scoring goals and he came up with two wonderful strikes," said Hughes.
"I had a great smile on my face at the start of the season when one or two people were knocking him for not scoring goals.
"I always said he would get well into double figures and that's him now on nine and long may that continue."
Hughes fears losing the Northern Irishman when his contract ends in the summer.
"He is a godsend and, as a club, we are working hard to keep him here and, when you see that talent in terms of putting the ball in the net, he is worth his weight in gold," said the manager.
Hughes thought the red card had changed the game in Caley Thistle's favour.
"It took us going down to 10 men and losing a goal to give us that intensity in our game," he suggested.
"But throughout the match I thought we dominated the game, but we lacked that killer instinct in the final third during the first half.
"The whole game changed on the sending off. They got their goal and it gave us the impetus to get on the front foot."