In Pictures: Keane's Celtic debut ends in defeat by Kilmarnock
Kilmarnock 1 Celtic 0
Robbie Keane's Celtic debut was marked with a new low for Tony Mowbray's reign as the club suffered its first defeat to Kilmarnock in 32 meetings.
On-loan Tottenham striker Keane missed two golden opportunities to score in either half and he was upstaged shortly after the break by Chris Maguire, who celebrated his own debut with a fine winner for Killie.
The result left Celtic 10 points behind Rangers in the Clydesdale Bank Premier League, but now having played the same number of games, while also giving their opponents' survival hopes a massive shot in the arm.
The outcome was eerily similar to Roy Keane's Celtic debut, which ended in a Scottish Cup shock at Clyde.
Namesake Robbie - who was treated to a hero's welcome by thousands of delirious Hoops fans after signing last night - was joined in Celtic's starting XI by fellow new boy Diomansy Kamara, while the fit-again Glenn Loovens and Lee Naylor were recalled.
Killie - who had lost 29 and drew only two of their previous 31 games with the Hoops - handed Tim Clancy a first appearance since September as well as unveiling Maguire.
It was thought Keane's signing would spark a rush of ticket sales for tonight's game, which had been rearranged after the big freeze claimed December's original fixture.
There was certainly a large travelling contingent, who greeted the arrival of their new idol in similar fashion to his midnight unveiling.
But it was Ki Sung-Yueng who took centre stage in a strong Celtic opening, blasting over in the fifth minute before seeing a free-kick tipped wide by Cameron Bell two minutes later.
Killie's Liam Kelly tested Artur Boruc from long range and Jamie Hamill poked wide from too tight an angle.
He then curled a shot straight into the arms of Boruc before Celtic almost scored against the run of play 20 minutes in when Garry Hay cleared Marc-Antoine Fortune's flick from Kamara's corner off the line.
Kamara, who had been virtually anonymous to that point, then bulldozed his way to the edge of the box before shooting wide.
Celtic were forced into a change on the half-hour when Andreas Hinkel suffered what looked a thigh injury and had to be replaced by Paul Caddis.
Keane's first attempt on goal immediately followed, the Republic of Ireland captain shooting wide from 15 yards.
He sent a cheeky chip over the bar five minutes later but Killie were still enjoying much of the ball, having taken confidence from Saturday's comeback against Dundee United.
Seven minutes before half-time, Keane should have scored but he sidefooted straight at Bell after bursting through one on one with the Killie keeper.
Celtic began the second period much as they had the first, with Fortune seeing a shot deflected behind six minutes in after turning well in the box.
But Killie broke up the other end and scored just over a minute later, Mehdi Taouil exchanging passes with Clancy inside the box and squaring for Maguire to finish low beyond Boruc.
Mowbray responded by bringing on Josh Thompson for Jos Hooiveld at the back before Aiden McGeady was booked for a touchline foul on Hay.
Landry NGuemo quickly followed for a lunging challenge on Craig Bryson.
Killie introduced Conor Sammon for Taouil just past the hour mark as the visitors began to press.
Keane volleyed Fortune's cross wide with 25 minutes remaining, though he was adamant the shot was deflected.
With 17 minutes left, Mowbray played his final card by throwing on Scott Brown for Naylor, the fit-again midfielder's first appearance for almost three months.
Killie also brought on new loan signing Rob Kiernan for Clancy.
Loovens glanced Kamara's corner past the far post before Scott Severin was also handed his Kilmarnock debut, replacing Maguire with 10 minutes to go.
Ki volleyed narrowly wide as Celtic's desperation grew.
Hamill missed the easiest of chances to put the game to bed when he shot straight at Boruc and Keane saw a second great opportunity go begging when the outstanding Bell saved with his feet moments later.
Brown headed straight at the keeper in the closing moments as Killie held on for a famous victory.