Martin O'Neill has returned to Celtic and will manage the team until the end of the season in a second spell as interim boss.
The 73-year-old is back in the role following the sacking of Wilfried Nancy earlier on Monday.
O'Neill won seven of his eight games in charge between Brendan Rodgers' dismissal and Nancy's appointment.
Celtic are at home to Dundee United on Saturday. They are currently six points behind league leaders Hearts in the Scottish Premiership.
In a statement, O'Neill told supporters there is still "a lot to look forward to" this season.
"I am really pleased, in fact, very honoured to be asked back to manage the team again and I am looking forward to getting back to work again with the players.
"I know we would all have hoped for things to have worked out differently under Wilfried and I personally want to wish him good luck with everything he does in the game. He is a fine man and I am sure he will go on and achieve success again, I have no doubt of that.
"For me, I've been asked to take this great job on again and my focus will be to try and get us back to winning ways if we can. We will need everyone right behind us.
"We still have a lot to look forward to and a lot to try and achieve, and myself, Shaun and the coaching staff and, of course, the players will be giving everything to achieve all the success we can for the club."
He will be assisted by Shaun Maloney and Mark Fotheringham as he was in his previous time in charge last year.
Stephen McManus will join the existing coaches, Gavin Strachan, Stevie Woods and Greg Wallace to oversee first-team matters.
Celtic move swiftly to bring back O'Neil after Nancy sacking
Nancy was sacked by Celtic after just 33 days in charge.
The Frenchman suffered six defeats in eight games. His final match was a 3-1 home defeat to Rangers on Saturday.
A club statement read: "Celtic Football Club announces that it has decided to terminate the contract of manager Wilfried Nancy, with immediate effect.
"The club thanks Wilfried for his efforts and wishes him and his family well for the future.
"Wilfried's assistants, Kwame Ampadu, Jules Gueguen, and Maxime Chalier, will also be leaving the club and they take with them our good wishes.
"The club can also confirm that Paul Tisdale has left his position as Head of Football Operations. We thank Paul for the contribution he has made in that role.
"A further update will be provided to supporters as soon as is practical."
Nightmare under Nancy
Nancy, 48, signed a two-and-a-half-year contract with the Scottish champions last month, joining from Major League Soccer side Columbus Crew.
Celtic's form slumped dramatically under Nancy after they had won seven of their eight games under interim manager O'Neill.
Nancy became the first Hoops boss to lose his first two games in charge before suffering a 3-1 defeat by St Mirren in the Scottish League Cup final and a 2-1 loss away to Dundee United.
It was the first time the club had lost four games in a row since 1978 under the legendary Jock Stein.
They won back-to-back league matches against Aberdeen and Livingston, but were outclassed by Motherwell at Fir Park and collapsed in the second half against Rangers.
'Right decision, but problems run much deeper than Nancy'
Sky Sports' 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.
Should Celtic have learned from Martin's Rangers spell?
Sky Sports' Alison Conroy:
Rangers took a risk when they appointed Russell Martin in the summer. He was not the fans' choice, but the Rangers hierarchy admired the way he set his teams up and the style of football he wanted to play.
The problem was that supporters turned on him almost immediately.
His football philosophy may have looked good on paper, but he had a squad of players who could not make his plans work.
Martin lasted 123 days.
Nancy's tenure has been even shorter. Is it fair to say Celtic should have learned something from their Old Firm rivals?
The Frenchman's philosophy and style convinced the board he was the right man to replace Brendan Rodgers, even though not all the fans were convinced.
Questions were raised about whether the players could execute his style of football. Sound familiar?
Something else that sounded familiar was post-match interviews.
After drawing with Dundee, Martin told Sky Sports: "It's great learning for the players. It's a completely different challenge for them today and one we'll be better for moving forward."
Fast-forward to Celtic's defeat to Rangers, Nancy insisted: "I see many, many good things. That's why the frustration is here because we deserve better and we have to stay together and everything's going to move forward."
At Celtic and Rangers, fans demand wins.
Martin found that out the hard way. It seems Celtic learned little from that era at Ibrox - and Nancy's ending wasn't any better.
- Dundee Utd (H) - Scottish Premiership - January 10
- Falkirk (A) - Scottish Premiership - January 14
- Auchinleck Talbot (A) - Scottish Cup - January 18
- Bologna (A) - Europa League - January 22
- Hearts (A) - Scottish Premiership - January 25 live on Sky Sports
