City had effectively been building a home waiting for Guardiola to move in and the man any club in the world would want in charge was finally ready to take up residence after leaving Bayern Munich.
What was not part of the plan was a slump in form that raises the possibility of Guardiola taking charge with City in the Europa League - a prospect that must now be considered as Manchester United closed to within a point of Pellegrini's fourth-placed side with a 1-0 win at Etihad Stadium on Sunday.
What has been happening to City's season to leave them in this perilous position?
This is an easy accusation to make and can be supported by the fact City have won only three games out of 11 since Guardiola's arrival was confirmed in an announcement in February - but is it actually true?
City have won a trophy - the Capital One Cup at Wembley against Liverpool - since then and it would be doing Pellegrini's squad a kindness to suggest they have only become indifferent and below-par since it became known Guardiola was on his way.
They have also made club history by reaching the quarter-finals of the Champions League for the first time with victory over two legs against Dynamo Kiev - but the signs of decline were in evidence well before 1 February.
Manchester City have not been at their best for any period since they began the season with five straight Premier League wins. They had already lost five league games before the Guardiola announcement was made.
City won their first three away games in the Premier League but only two of the next 11 and have not won back-to-back league games in five months. The rot set in long before they started getting ready to put Guardiola's name on the manager's office door.
It is hard to believe City's players have been dwelling on the Spaniard's arrival during games and in an uncharacteristically prickly post-match media conference after the United loss, Pellegrini was at pains to deflect suggestions Guardiola was providing an unwelcome distraction.
He said: "I don't think I have lost any control. The attitude of the team was excellent. I repeat - I am happy with attitude of the players."
Guardiola's impending arrival may have focused minds on City's shortcomings - but this was a team showing signs of struggle and decline well before he was confirmed as Pellegrini's successor.
Manchester City are only one point ahead of Manchester United and West Ham even though they still lie fourth, and 15 points behind leaders Leicester City having played a game less.
The rest of the season is not about winning the title but finishing in the top four - an embarrassing state of affairs for a club with City's aspirations and scale of ambition.
And perhaps the most tell-tale statistic of why they are where they are comes in their record against the current top eight.
City have played 11 matches against them, winning only one, drawing three and losing seven for a meagre return of six points. They also lost heavily home and away to ninth-placed Liverpool.
Their total of 51 points from 30 games is a grim reflection of their struggles throughout the season.
In 2011-12 they had 70 points at this stage of the season, the following year they had 62 points and 67 when they brought the title back to Etihad Stadium in 2013-14. Last season they had 61 points after 30 games.
Guardiola's first priority will be to keep everything crossed that stumbling Manchester City can keep it together long enough to make sure they finish in the top four - or win the tournament of course - to ensure they are in next season's Champions League.
He inherits a squad that is guilty of under-achievement this season, even though they have won the League Cup and remain in the Champions League.
City's buying policy has been flawed to such an extent in recent years that they still rely heavily on the backbone of the side that won the club's first title in 44 years in 2012 - namely keeper Joe Hart, captain Vincent Kompany, midfield pair Yaya Toure and David Silva, as well as striker Sergio Aguero.
Around that key group others have not achieved, while there are problems even with those players. Kompany is 29 and suffering from acute calf problems, Toure is 33 in May and was sold by Guardiola at Barcelona, while even the peerless Silva is now 30 and has been troubled for many months by an ankle problem.
Guardiola will be happy with Hart in goal and will want to build around 24-year-old Kevin de Bruyne, who has been such a big miss through injury in recent months. He will also want Raheem Sterling, a £49m summer signing from Liverpool, to justify that fee.
He may well target Everton's 21-year-old  John Stones to give youth and authority to a central defence that is still holed below the waterline by Kompany's many absences, with Martin Demichelis looking every day of his 35 years and more against Manchester United and both Nicolas Otamendi and Eliaquim Mangala not showing a return on a combined investment of around £70m.
Wilfried Bony was signed for £28m from Swansea City as the likes of Edin Dzeko and Stevan Jovetic were allowed to depart, but it is hard to see a striker who lacks mobility surviving under Guardiola.
In defence, Bacary Sagna is 33 and coming towards the end of his Premier League career, while Pablo Zabaleta is 31 and Aleksandar Kolarov is 30 - this is a City team that has been allowed to grow too old, with poor signings exacerbating the problem.
Jesus Navas has been poor and Samir Nasri has provided moments of magic and mediocrity.
So Guardiola's priorities will be to freshen up and introduce young blood into a stale squad, while also bringing authority to all parts of the pitch, especially in central defence and in giving attacking support to Aguero.
For all City's ability to flex their financial muscle, he will find that a lot easier to do with them in the Champions League rather than the Europa League.
The 62-year-old Chilean has one trophy in his locker in his final season but there will be no Premier League title - he is relying on the Champions League to give the campaign a flourish because a top-four finish should be a given, not a source of celebration.
Pellegrini has brought Manchester City to the last eight of the Champions League and the draw has been relatively kind as they avoided the big three of Barcelona, Real Madrid and Bayern Munich to be paired with Chelsea's conquerors Paris St-Germain.
It is a tough task - one a City squad firing on all cylinders might just fancy - but right now they are seriously underpowered.
Pellegrini has been dogged by those injuries to Kompany, De Bruyne and Silva and matters got worse on a bad day against Manchester United as Sterling and keeper Hart suffered injuries that mean they may miss the PSG games.
Hart's calf injury looked a serious problem as he was taken off on a stretcher after rescuing Demichelis from a dreadful back pass.
Sympathy will be in short supply if City cite injuries given their resources - but they are facing real problems.
City will be in dreamland if they can get into the Champions League's last four but it is hard to see them, in their current state, troubling the trio of superpowers in this competition.
It may well be that the League Cup and a place in the top four will have to suffice before Pellegrini takes his leave.
When Manchester City announced that Pep Guardiola - the world's most coveted and celebrated manager - would succeed Manuel Pellegrini in the summer, there was not a cloud on the club's horizon.