It's United we're talking about. Given the particularities and the history of our club, i'd pick a great manager every single time i had to make the choice. With patience, a great manager would create a great side. It has never worked the other way around for us.
Both Real Madrid and Barcelona would probably keep being successful with average managers, for a while, but for different reasons. The former have turned the places in their first-team into the pinnacle in a footballer's career, especially as the Spanish speaking countries are concerned. Barca have their own modus operandi, they know what type of players they want to produce. Then you have financial behemoths (with a bit of financial doping, as it seems) like City and Chelsea who can afford to buy their way into a higher status in the football world.
But it's not like that the aforementioned clubs didn't go through their own dark periods or that they didn't benefit from picking great managers at crucial crossroads in their history. Real Madrid went from 3/5 CL titles to not winning a single knock-out tie in Europe for several years under the guidance of Queiroz, Luxemburgo, a washed-up Capello, Schuster, Juande Ramos and Pellegrini. People keep pointing at Barcelona under Luis Enrique and Valverde but where would Barcelona even be now without the great Cruyff or without Pep leading their most recent team to unprecedented levels. Mourinho, at his managerial peak, used Abramovic's money to create a side with such solid defensive foundations and strength of character which all the average managers that followed after him kept milking for many years. And you seriously can't tell me that Mancini/Pellegrini's City were as intimidating and revered for the quality of their football as Pep's. In team sports, what constitutes a "great" squad is more than having 8-9 primadonnas in your ranks. It's about tactics, vision, good planning and building team character. You can take the short-cut (money) to assemble the potential but in order for that potential to be fulfilled, you need a great manager. Then the Valverdes, the Enriques and the Di Matteos can win some silverware for themselves. But when you ask these guys to do the heavy lifting and give the team an identity, they will stumble and fall on most occasions. I guess it's easy to see in which category United belongs atm.
United's successful periods always revolved around a family environment with the manager as the leader. As a club, we have always depended on the manager for guidance. We don't have a distinct way of playing football (other than being entertaining and attack) like Barcelona, we can't monopolize the local talent pool like Bayern or Juventus do, we never were as attractive as Ream Madrid and we can't afford to spend maniacally like City and Chelsea. We've always taken pride in being a traditional football club like our bitter rivals in Merseyside. Look what a great manager has done for them. Now, close your eyes and imagine them being coached by any other manager except Klopp.
It's similar to saying that someone like Solskjaer (who i suppose for some should be blameless because of the poor quality of our squad) would have been as successful as Sir Alex if he had taken over at the start of 1992/93 season when Ferguson finally had his team ready to win the title. You're fooling yourselves. That team and the ones that followed had Fergie's signature from top to bottom. An average manager might have squeezed the squad up until the mid naughties and that would be it. We would still have needed Fergie's character, his vision and his ability to adapt.