Robbie Keane's Celtic debut turned sour on Tuesday when the Glasgow side were beaten 1-0 at Kilmarnock to leave them 10 points behind Scottish Premier League leaders Rangers.
Ireland skipper Keane, signed on loan from Tottenham Hotspur on Monday, was in the starting line-up along with fellow new signing Diomansy Kamara, but was left frustrated.
Chris Maguire gave Kilmarnock the lead seven minutes into the second half and they held on for their first home league win against Celtic for nine years.
Keane, the former Inter Milan, Leeds United and Liverpool striker, had several chances to open his Celtic account but was denied by keeper Cammy Bell.
"It's not the result we wanted," Celtic manager Tony Mowbray told the BBC. "We needed to hit the ground running with the new signings, but it didn't quite happen tonight."