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.