Celtic manager Brendan Rodgers and Manchester City have shared histories. It was City who stole the 2013-14 Premier League title once Steven Gerrard's slip and a 2-0 defeat to Chelsea derailed Rodgers' Liverpool team. Previously, in the summer of 2013, he was on a short list of candidates for a vacancy that eventually went to Manuel Pellegrini.
The pace of change in football means Rodgers, for whom the anniversary of his Liverpool sacking will arrive next week, is now in Glasgow while City are led by Pep Guardiola, the manager the club truly wanted back when the Ulsterman was a contender to replace Roberto Mancini.
"I have watched and worked against Manchester City the past few seasons," Rodgers told reporters on Tuesday, ahead of City's Champions League Group C visit to Celtic. "The level of their play has gone up since Pep has come in."
And so, on Wednesday, a team four points clear of the English Premier League will visit an opponent holding the same margin at the top of the Scottish Premiership table, though that's where the similarities end. The two clubs are oceans apart in scope. City turned to Guardiola, the world's most admired coach, to realise their project of winning the Champions League. Celtic, by far the biggest fish in the piffling pond Scottish football has become, have little hope of again lifting the trophy the club won in 1967.
"Celtic is a huge club but it's night and day different in terms of the leagues and resources," Rodgers said. His current club is merely happy to be back in the group stage as the financial rewards will insulate its status in Scotland. It would, though, be highly damaging if Celtic were proved to be there for nothing other than making up the numbers. Another defeat like the 7-0 suffered at Barcelona's Nou Camp a fortnight ago and Rodgers' reign takes a serious hit in terms of credibility. A loss of that magnitude was an embarrassment that dimmed the glow of beating Rangers 5-1 the previous weekend.
"We won't be as passive as we were in the last game," Rodgers promised. "Apart from Barcelona we have been really aggressive in our game. The most important thing was to learn from it."
Rodgers also spoke of the "need to ensure we're committed" and that may be exemplified by some players who should have something to prove to City. Three Celtic players, defenders Kolo Toure and Dedryck Boyata and winger Scott Sinclair, are former players cast aside by the club's acceleration in development while another, winger Patrick Roberts, made the move north in January in an 18-month loan deal.
Roberts, still just 19, has been given permission by his parent club to play against them for his temporary home and has the chance to show that he is worth the £12 million they paid Fulham in the summer of 2015. At Celtic Park, he was won admiration for his speed and attacking flair.
There are other shared themes. Toure's younger brother, Yaya, has been banished from City's Champions League squad while agent Dimitri Seluk wages open warfare with Guardiola. In the summer, Rodgers turned to Kolo (who he also signed for Liverpool in 2013) to Glasgow given his experience of grand occasions and he was Liverpool's best defender in May's 3-1 Europa League final defeat. That said, his 35-year-old legs might struggle with the fleet-footed nature of City forwards like Sergio Aguero, Raheem Sterling and Leroy Sane.
Boyata and Sinclair had markedly similar careers at the Etihad. Both players came through youth systems (at City and Chelsea respectively) and ended up merely serving the club's quota of homegrown players, receiving the reward of very little first-team football. They each made 13 Premier League appearances for City but have found differing fortunes north of the border. Boyata has struggled with injury and will miss out on Wednesday while Sinclair has made himself the most potent player in Scottish football.
City loaned and then sold Sinclair to Aston Villa, where he suffered Premier League relegation last season, but he has scored in each of his six league matches so far for Celtic. "He always has a hunger to work hard and to score," said Rodgers after Sinclair again notched during a 6-1 defeat of Kilmarnock. In Manchester, Sinclair was far better known for being the partner of TV star Helen Flanagan, an actress on Coronation Street, than for his football.
"It'll be great to see some old friends but for me it's just another game," Sinclair said on Monday. "I don't have anything to prove."
Celtic will be backed by the usual wall of noise that greets a European night at Celtic Park. Harnessing that crowd against Manchester City might be the best chance of getting an unlikely result as Rodgers and his players seek to prove their own credentials.