The World Game looks at who faces the biggest rebuilding job - Mourinho at United or Guardiola at City?
Fans
After the sterility and frustration of the Louis van Gaal era, even after winning the FA Cup, United fans are in need of a lift. The high-profile Mourinho can provide that.
His appointment may not be universally welcomed by supporters, as some of his past histrionics are an issue for some, but large swathes of the United fanbase have been calling for him to be installed for months.
If results come, Old Trafford is sure to be re-energised, at least in the short term.
Guardiola already seems to have all City fans onside, with some even planning welcome parties.
League positions
United have finished seventh, fourth and fifth in the last three English Premier League seasons and have the most ground to make up.
Yet they have only just missed out on a UEFA Champions League position on goal difference and they won the FA Cup, indicating they still have a competitive squad.
If the Mourinho effect quickly rubs off United could be challenging again in the not-too-distant future.
Meanwhile, City expect to be challenging now. Last season's fourth place was a huge disappointment.
Fresh blood will arrive but City should be able to climb the table if Guardiola can simply get them playing to their potential.
Transfers
Neither club should struggle recruiting this summer. Both managers will be backed financially with United's commercial power continuing to bring in record revenues while City's resources are almost limitless.
United cannot offer Champions League football, but the lure of playing at Old Trafford and for Mourinho will carry weight.
City may not have United's history, but they do boast Champions League opportunities and there will be no shortage of top players wanting to play for Guardiola.
They could strengthen in many positions, although wholesale changes are unlikely.
Keeping key players
United may have to combat renewed attempts from Real Madrid to prise away David de Gea.
The outstanding goalkeeper is the player United can least afford to lose and retaining him will be one of Mourinho's top priorities.
There are no other United players quite in the same bracket, although it may do them no harm to offer the experienced Michael Carrick a new deal.
Despite last season's under-achievement, City are still in a strong position when it comes to retaining players.
Sergio Aguero is the jewel in their crown but, despite expressing a desire to return to South America in future, he appears fully committed to the cause.
Yaya Toure is likely to receive offers from elsewhere, but he is now a declining force and may be allowed to leave if a replacement is found.
Youth
It seems remarkable that just a few months ago the famed United youth system that produced the Class of 92 was being criticised for drying up.
Since then the likes of Jesse Lingard, Marcus Rashford and Cameron Borthwick-Jackson have shone.
Their emergence was one of the plus points of Van Gaal's regime and probably puts United ahead of City in this regard at present.
City so far have just Kelechi Iheanacho to show for their much-trumpted £200 million ($407 million) academy.
Yet these are still early days in what is a long-term project at the Etihad Campus.
With a number of highly-rated players in their system, particularly in the 14-16 age bracket, the fruits of City's work could be seen in the years ahead.
Summary
Mourinho seems to have the bigger job to do to rouse a fallen giant, yet his past record, cash reserves and youthful squad nucleus suggest he can get there quickly.
Guardiola is being viewed more like the final piece in the City jigsaw - his arrival alone should prompt an upturn in fortunes in an under-achieving squad.
That said, their group is an ageing one and a process of renewal needs to begin.
