Jordan Jones showed his loyalties still lie with Kilmarnock as he wrecked Jermain Defoe's Rangers debut by firing Steve Clarke's side to a 2-1 victory at Rugby Park.
The Northern Ireland winger is set to join Steven Gerrard's squad after signing a pre-contract earlier this month.
But he was trusted to start against the side he will be joining in the summer and repaid his manager's faith with the goal that sees Clarke's men leapfrog the Light Blues into second place.
It took Defoe just 12 minutes to open his Rangers account but it was a mostly frustrating night for the former England frontman.
The 36-year-old did not even have much time to saviour his strike as a Joe Worrall blunder handed Eamonn Brophy the equaliser eight minutes later.
Having worked so hard to draw level with Celtic before the winter break, Gerrard's side now drop back to third.
The Rangers faithful have been licking their lips in anticipation ever since the news broke of Defoe's arrival on loan from Bournemouth - and it did not take him long to get the travelling supporters off their seats.
Killie goalkeeper Daniel Bachmann spilled an Andy Halliday drive but was up quickly to block James Tavernier before another follow-up from Ryan Kent was kicked off the line by Stuart Findlay.
The ball appeared to deflect off Steven Davis' hand but Defoe did not hang about for a whistle and tapped home for the perfect start to his Rangers career.
Bachman was booked for remonstrating over Davis' part in the goal but Defoe lapped up his moment as he ended a 298-day draught stretching back to Bournemouth's 2-2 draw with Watford last March.
Alfredo Morelos has had no such trouble finding the net of late but he saw a header from Kent's corner come back off the left post eight minutes later.
And from coming within an inch of being 2-0 up, Rangers suddenly found themselves pegged back after Joe Worrall's moment of madness.
The on-loan Nottingham Forrest centre-back was caught dawdling as he tried to bring the ball out from the back.
But he took his eye off the ball and Brophy needed no second invitation as he stole possession before racing away to beat Allan McGregor with a cool finish.
Neither Worrall nor his defensive partner Gareth McAuley looked comfortable on Killie's plastic pitch and the constant pressing of Clarke's front three only increased their anxieties.
Rangers survived until the break as they grabbed a much needed moment of respite from the hosts' constant harassment.
Morelos, frustrated by his lack of first-half involvement, used his hands to punch a Tavernier cross into the net three minutes after the restart but was not fooling referee Don Robertson, who flashed yellow at the Colombian.
And just as Gerrard's team looked to be regaining their composure, another calamitous mistake cost them the points.
Kent fired the ball straight to Jones in the 66th minute and the Northern Ireland international drove 40 yards upfield before firing a low strike from 25 yards past McGregor.
With his future manager watching on gloomily, the winger darted straight for current boss Clarke to thank him for his recent support.
Jones had angered the Killie fanbase with an ill-thought tweet when announcing his Rangers move but there was only cheers for the match-winner as he was replaced in stoppage time.
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.
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."
