With a combined age of 66, Rio Ferdinand and Nemanja Vidic are edging towards the end of their careers.
They have been Manchester United's first-choice centre-half pairing since Vidic arrived to join Ferdinand at Old Trafford in 2006.
They've won five Premier League titles together and the Champions League in 2008. They also helped United reach two other finals in 2009 and 2011.
In their pomp, they were considered the best central defensive partnership in the world—with Ferdinand's class and intelligence a compliment to Vidic's bark and bite.
But they can't go forever.
Ferdinand's niggling back injury and Vidic's knee problems have both been managed carefully over the last two years.
But last season, they only started together 10 times.
Both were sidelined for United's 1-1 draw with Southampton on Saturday, and there will come a time when David Moyes is forced to build a new defence permanently.
In no particular order, here are the top six candidates.
At a snowy Upton Park in November 2010, Jonny Evans' undoubted promise lay in tatters.
Destroyed by Carlton Cole during West Ham's 4-0 win in the League Cup, Evans was hauled off in disgrace before the final whistle.
But he has slowly rebuilt his United career to reach a point last season when he was preferred to Vidic for the Champions League tie with Real Madrid in the Bernabeu.
In the last two seasons, he's appeared in 50 of the 76 Premier League games, which is proof of how highly Sir Alex Ferguson came to rate him.
Still only 25, it will be Moyes who sees the best of him.
If Moyes decided to go English in his search for successors to Ferdinand and Vidic, he'll likely find that the best two prospects are already at Old Trafford.
Chris Smalling is only 23 but is already a full England international. Phil Jones, two years Smalling's junior, is too.
Neither have been able to hold down regular places for club of country, but it's still early days in terms of their respective careers.
Jones, in particular, might find his path to a regular spot at centre-back blocked by his own flexibility, having filled in at right-back and in midfield during his time with United.
Lower down the pecking order, England Under-21 internationals and 2011 FA Youth Cup winners Tom Thorpe and Michael Keane are waiting in the wings.
The Belgian is comfortable at left-back—another area Moyes will have to strengthen—and centre-half. He has been one of the Premier League's standout defenders in the last two seasons.
At 26 years old, he won't come cheap, but it wouldn't be the first time United have raided Tottenham for one of their stars.
If Champions League football doesn't return to White Hart Lane next season, Vertonghen will have a decision to make.
Looks a little like Jurgen Kohler and plays a lot like Franz Beckenbauer. It's not a bad combination for a centre-half.
At 24, Mats Hummels has made himself a first-choice pick for Germany and last season's Champions League finalists Borussia Dortmund.
A lack of pace is beautifully masked by his reading of the game and his composure. Also, he's as comfortable strolling out of defence with the ball at his feet as he is clearing a corner.
United would likely have to battle Barcelona if he ever did decide to leave Dortmund.
Moyes' scouts could also do a lot worse than taking a closer look at Hummels' centre-back partner Neven Subotic, but there's no guarantee that the Serbian international would want to leave Westfalenstadion either.
Benedikt Howedes came through the youth system at Schalke and has progressed to become a full German international.
The 25-year-old was part of the Schalke side that was battered 4-1 at Old Trafford in the Champions League semi-final second leg in 2011.
But he was also a part of the Germany Under-21 side—alongside Manuel Neuer, Jerome Boateng, Sami Khedira, Mesut Ozil and Hummels—that won the 2009 European Championship, having thrashed England 4-0 in the final.
He only signed a contract extension this summer, so United might have to wait at least another year if they want to make a move.