Others say Mourinho has lost his magic after what he has described as his "worst" season as a coach in the trophyless year at Real Madrid. Also, can his relationship with Chelsea owner Roman Abramovich, who sacked Mourinho six years ago, be repaired? Whatever happens, it will be an entertaining ride under Mourinho.
It will be no less fascinating to see how Moyes gets on at United.
Ferguson has urged fans to get behind his fellow Scot but what will be the reaction if United loses its opening two games? How quickly will Moyes be able to gain the respect of his players? Can Ferguson hold himself back from meddling in team affairs from his position up in the director's box?
"He will have his own ideas and that's good," Ferguson has predicted. "He will be fine."
Moyes will have to act quickly to sort out the futures of Wayne Rooney and Rio Ferdinand this summer and maybe sign the world-class playmaker many United fans have been craving for years, with Paul Scholes having retired and the team's central midfield still lacking creativity at times.
With Ferguson gone, there hasn't been such uncertainty swirling around Old Trafford since the late 1980s and opponents will be keen to seize on it.
Particularly Man City, which will be desperate to reclaim the Premier League trophy from its neighbor and rediscover the consistency and attacking spark that was lost in the final 12 months of Mancini's 3 ½-year reign.
City demonstrated in the way it beat United 2-1 away in the derby last month that, on its day, there is no better team in England. And with Mancini gone, the likelihood is that the cracks which reportedly developed in the squad in recent months should heal.
Is Pellegrini the man to restore City's fortunes, though? He has never coached in the Premier League and was overwhelmed by all the egos in Real Madrid's squad during his one season at the Spanish giant in 2009-10. Yet, his record at Villarreal and Malaga, his current club, is impressive and has been hailed for his skills in man-management.
It is another appointment that could go either way, adding to the intrigue next season.
There should be plenty of new faces among the playing squads, too, with the transfer kitties of clubs boosted by a 70 percent increase in TV revenues announced last summer and taking effect from the start of next season. The financial pull of the Premier League has never been so strong, especially with many top clubs in Spain and Italy still suffering from the impact of the economic downturn.
The recently completed Premier League campaign had one of the most anticlimactic finishes in years, but Ferguson has just ensured that the new season won't be short of excitement.