Steven Gerrard admits Rangers undid all of their hard work against Celtic after shooting themselves in the foot against Kilmarnock.
Jermain Defoe marked his Gers debut with a goal inside 12 minutes at Rugby Park.
But Joe Worrall's nightmare blunder allowed Eamonn Brophy to level for Steve Clarke's side, before Jordan Jones pounced on a misplaced Ryan Kent pass to hand the side he will be joining in the summer a 2-1 defeat.
The Light Blues were hoping to reproduce the kind of display that saw them beat Celtic for the first time in the league since 2012 in their final game of 2018.
But having worked so hard to draw level with the Hoops, they now find themselves back to third after freezing on their return to action following the winter break.
Asked if his team had let their Old Firm spoils slip with their fourth defeat of the campaign, Gerrard said: "I think so.
"We got a lot of praise for beating Celtic. We won every single key individual battle in that game.
"I'm struggling to find one of them tonight. So we deserve the criticism.
"It's even more painful because we had so much control in the first 15 minutes of the game. One moment had a huge effect on us and that's a concern.
"It is a difficult one to take. A huge mistake gave Kilmarnock the opportunity to get back into the game.
"That moment affected the whole team and the whole atmosphere in the ground. We never recovered.
"The two big moments which have gone against us, we have gifted Kilmarnock.
"There was Joe's moment for the equaliser and then a square ball in the second half which sees us get done with a counter attack.
"We backed off Jordan and let him onto his favoured foot. A player of that ability is going to punish you from 20 yards.
"This was a game we should have won comfortably. But when you gift teams two big moments, more often than not you get punished. So you don't deserve to win."
Defoe bundled home from close range but his opener was levelled up by Brophy eight minutes later after Worrall dawdled on the ball trying to build from the back.
And Jones showed he could handle the pressure of facing his future employers as he drove forward from Kent's wayward pass to blast past Allan McGregor after 66 minutes.
"Jermain gets his goal, that's what he does and what he will do – I'm sure he will go on and score many more," said Gerrard.
"Him and Steven (Davis) will get better and sharper with time. I have to be careful on their load at the moment because they haven't played much football."
Clarke's side leapfrogged Rangers into second place and the Rugby Park boss was overjoyed with his match-winner: "Jordan showed he's a big player for us.
"He got the move to Rangers that he wanted and he can be a big player for us in the second half of the season.
"It's nice to see him get a goal as he goes on that run often and it doesn't always end with a goal.
"Sometimes fans get excited when things like this happen. I know Jordan well and the qualities he has.
"He's been great for the club for two-and-a-half seasons and I look forward to him being great for this club until the end of the season."
