The Parkhead side travelled to Fir Park on the back of a momentous 2-1 Champions League victory over AC Milan at home on Wednesday but found the Border side hard work.

Fabian Yantorno's 30-yard free-kick after 36 minutes gave Gretna a shock half-time lead and Gordon Strachan's men struggled until five minutes to go when Killen, starting his first match since signing from Hibernian in the summer, headed home an equaliser with his first goal for the club.

Then, in the 90th minute, Scott McDonald, who had fired in a late winner against Milan in midweek, drove home from 12 yards to give Celtic all three points and take them back to the top of the league.

The New Zealand international, celebrating his 26th birthday today, said: "We didn't really play too well but we can't use the AC Milan game as an excuse.

"We know we weren't good enough.

"We treat AC Milan the same way we treated Gretna but we were just not at the races.

"Gretna were well-organised and hard to break down.

"Sometimes when you are not playing well you have to put in hard work and effort."

Killen enjoyed his goal - a looping header from an angle - and admits the result could be significant later in the season.

He said: "The ball came over and I had to put it over a few bodies to try and guide it in but it was an attempt at goal, definitely.

"The win could be a deciding factor at the end of the season but hopefully we can go from strength to strength now."

Celtic manager Gordon Strachan's delight was tempered by the news that Paul Hartley, stand-in at right-back against Gretna, will be out for at least three weeks after picking up a hamstring injury.

The former Hearts player will miss Scotland's Euro 2008 double-header against Ukraine and Georgia next week and the Old Firm game at Ibrox in a fortnight.

The midfielder was playing in the unfamiliar position in the absence of three injured right-backs - Mark Wilson, Jean-Joel Perrier Doumbe and Paul Caddis.

The Scotland midfielder was substituted after 18 minutes and Strachan said: "Paul has run about for ever this season but 15 minutes in that right back position and he pulls a hamstring. That is usually a three-week job.

"Gary Caldwell and Stephen McManus picked up knocks but they won't be pulling out of any Scotland squad."

Shell-shocked Gretna boss Davie Irons struggled to contemplate the loss of a match that looked like giving them their second win of the season.

He said: " To have three points snatched away is disappointing to say the least.

"I thought we had something from the game.

"The boys were magnificent and their effort and commitment was clear for everyone to see.

"They had not caused us too many problems although we had our backs to the wall.

"It's just hard to take losing at such a late stage of the game."