Martin O'Neill admits winning the Scottish Premiership will be a "difficult task" following Wilfried Nancy's 33-day reign, as he prepares for a second spell as interim boss.
The 73-year-old first returned on a temporary basis after Brendan Rodgers' departure in October, and is back again following .
He won seven of his eight games during that first spell, with Nancy going on to lose six of the following eight before he was dismissed.
After leaving the Hoops level on points with Hearts, he returns to a side trailing the Premiership leaders by six points.
"I think it's going to be difficult, make no bones about it. There are fewer games to play now than before, and Rangers must have gained a psychological advantage there, and that's over us," O'Neill said.
"We still have to catch Hearts, and the way that Motherwell played the other evening against Celtic, they might well be the best footballing side in the league.
"Hearts have had momentum all season. They've been able to win some big games, so it is a difficult task for us.
"The lads are in decent spirits, we had a bit of a meeting and honestly they genuinely feel as if they're in good heart.
"Naturally disappointed with the recent results and the way things have gone, but I'm just hoping that they're up for the battle ahead, and I'm sure they will be."
O'Neill will be assisted by Shaun Maloney and Mark Fotheringham in what is his third spell in charge at Parkhead.
Stephen McManus will join the existing coaches, Gavin Strachan, Stevie Woods and Greg Wallace to oversee first-team matters.
"I wonder whether I would have the capacity to go and carry on, and also I'm 112 next birthday, so I think time is kind of running out for me," he added when asked if he would consider staying beyond the end of this season.
"I'd like to try and do well in and who knows what the board might think, but I would not be, and I'm serious about this, I wouldn't be thinking any longer than the end of the season for me, regardless of what happens."
Celtic have already signed until the end of the season, but O'Neill's determined to strengthen the squad much further this month.
"We definitely need to supplement the squad, absolutely no question about that.
"Shaun has been working in the background in certain things, I think he would get to know some players.
"It's up to me as quickly as possible, because eventually, I would like to be sitting here in front of you and saying 'listen, all these players are my choice', that's what I really like to do.
"I don't have a great deal of time. I will rely on Shaun, Martin and Steve and their opinion on players, but eventually, it'll be down to me as much as anything else.
"If they don't do well, I'll blame Shaun, and if they do brilliantly, I shall take all the credit!
"I'll reiterate, we need the players in, and as quickly as possible.
"If the board were to listen to me, then we would have about 56 players.
"I will try and narrow it, we definitely know what positions that we need to strengthen in and that's what we're going to try and do."
Your Site' Chris Sutton:
"It's a bit like the Ruben Amorim situation at Manchester United but over a shorter period.
"He was fixated on changing Celtic's style, moving to a 3-4-3, playing one particular way. He just wasn't adaptable and for Celtic to lose six and ship as many goals as they did - it was kamikaze stuff from Nancy.
"You want to give a manager time, but I think Celtic have acted decisively because there is a league title to win.
"The players have to take their share of responsibility. Poor form has not just been under Nancy; there were issues at the start of the season.
"Brendan Rodgers wanted more quality brought to the club, that didn't happen. It was an unsavoury end. Things brightened up a bit under Martin O'Neill, he did a brilliant job, but the team weren't play particularly well. Nancy came in and tore everything up, but the problems do run much deeper than Nancy.
"Nobody knows what Paul Tisdale was doing at the club. Dr Do-little.
"He had a large say in bringing Nancy in, and if he had a hand in recruitment over the summer that was also poor.
"It's the right decision for him to part ways. Now Celtic are under pressure because this squad needs strengthening in January and quickly."
