Brendan Rodgers has left Celtic with more than two years left on his contract and in the midst of the most challenging Scottish title race for years.
The lifelong Hoops fan declared he had taken Celtic as far as he felt he could after leading them to seven consecutive domestic trophies.
Here, Press Association Sport looks at the reasons behind Rodgers' departure.
Transfer Restrictions
Rodgers was open about his frustrations over Celtic's transfer strategy in the summer, with the failure to sign John McGinn from Hibernian sparking a public airing of his grievances. Aston Villa stepped in after Celtic had several bids rejected and Rodgers told reporters he "150 per cent" agreed with suggestions they should have pursued the deal more aggressively.
Rodgers also struggled to get in a centre-back until the end of August when they signed Filip Beknovic on loan from Leicester. By that stage Celtic were out of the Champions League after losing to AEK Athens in a game in which Dedryck Boyata refused to play despite Rodgers' assertion the want-away centre-back was fit.
After the defeat, Rodgers told BBC Sport: "It's pretty obvious, you have to keep progressing and keep getting stronger. You only need to look at Liverpool, who got to the Champions League final and invested the money they had to be stronger. It's not rocket science. You have to always guard against becoming complacent and the way you do that is by adding to the squad."
Celtic's January signings were largely short term – loan deals for Timothy Weah, Oliver Burke and Jeremy Toljan – or 'project' players, including a Ukrainian winger, Maryan Shved, that Rodgers admitted he knew little about.
The make-up of Celtic's defence next season looks extremely uncertain with Boyata and Mikael Lustig's contracts set to expire and Benkovic certain to be reunited with Rodgers, and the summer activity will be a challenge.
European glass ceiling
Celtic have failed to build on successive Champions League qualifications and, for a second season running, have gone out in the last 32 of the Europa League. After a 3-0 aggregate defeat by Valencia, Rodgers said last week: "It's maybe difficult for us. But we can't give up hope. You saw the team play very, very well tonight. We have to look to qualify for European competition again next season and try to progress."
Heavy defeats by the likes of Barcelona (7-0) and Paris St Germain, who scored 12 goals in two games against the Hoops, sparked criticism of Rodgers' tactics, but the financial chasm between the Scottish league and the richest leagues in Europe is only going to widen in the near future.
Lure of the Premier League
Rodgers has decided to swap the chance to achieve a record 10 Scottish titles in a row for the opportunity to make his mark in the English top flight following previous spells with Swansea and Liverpool, where he narrowly missed out on the title.
His new club showed what was possible when winning the league in 2016 and even taking them into the top six would enhance his chances of landing a bigger job in subsequent years.
After winning seven trophies in a row in Scotland, he may have felt he was best to go out on a high, even if the timing upset the Celtic fans.
