Sir Alex Ferguson built some great Manchester United teams.
He had the benefit of match-winners. Eric Cantona, Ryan Giggs, Ruud van Nistelrooy and Robin van Persie. Peter Schmeichel, Mark Hughes, Ole Gunnar Solskjaer and Cristiano Ronaldo.
They helped him win 13 Premier League titles, five FA Cups, four League Cups and two Champions League titles.
They were responsible for some of the defining moments of Ferguson's reign. Giggs' free-kick against Blackburn in 1993. Cantona's winner against Liverpool at Wembley in 1996.
Solskjaer's late intervention against Bayern Munich in 1999. Van Persie's volley against Aston Villa in 2013.
But ask Ferguson what the key to his great teams was and the answer would always be the same. It wasn't the match-winners. The Cantonas, the Ronaldos and the Van Persies. No.
His first Premier League champions in 1993, the first double winners in 1994, the 1999 treble winners and the 2008 European champions all had one thing in common.
A strong and settled centre-back pairing.
That, according to Ferguson, is the foundation on which all great teams are built. He had Steve Bruce and Gary Pallister. Ronny Johnsen and Jaap Stam. Rio Ferdinand and Nemanja Vidic.
All terrific defenders in their own way. But most successful as a pair.
Louis van Gaal has been tasked with building United's next great team. And another great centre-back partnership to go with it.
Ferdinand and Vidic will both leave Old Trafford this summer. Jonny Evans, Chris Smalling and Phil Jones are talented young footballers, but not one can claim to have progressed this season. Michael Keane and Tom Thorpe are England Under-21 internationals but haven't got a Premier League start between them.
When Van Gaal sits down to survey his new squad and decide on the areas he needs to strengthen, centre-half will be high on his list.
He has options.
There are experienced, more expensive players like Mats Hummels or Ezequiel Garay. Or younger ones like Stefan De Vrij or Raphael Varane.
Van Gaal will hope there's another Bruce and Pallister or Ferdinand and Vidic in there somewhere.
It's the Dutchman's job to build a team as successful as Ferguson's have been. The Scot's sides became famous for their flair and invention. For the goals they scored and the highlights they created.
But they were always built on solid foundations. A strong goalkeeper and two centre-backs who understood and complemented each other.
Van Gaal will find he has plenty of work to do when he finally settles into his office at Carrington later this summer. Ferguson would tell him to start from the back.
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