Rio Ferdinand and Nemanja Vidic have had a good run.
They've won five Premier League titles together and stood side by side in three Champions League finals, winning one.
They've earned a place as one of the most formidable centre-back pairings United have ever had, alongside Steve Bruce and Gary Pallister and Ronny Johnsen and Jaap Stam.
But time waits for no man. And with a combined age of 67, it's time for Ferdinand and Vidic to go their separate ways. That's not to suggest they need to pack their bags and head for the Old Trafford exit.
To lose that much experience with one swing of the axe would be foolish. It's simply time for them both to find a new partner. Like a couple who've been together so long that the spark has disappeared, it's time for a change.
It's been coming for a while.
Not since the 2008-09 season have Ferdinand and Vidic started together in close to half United's games. That year, they started 32 of United's 66 games with a win percentage of 63 per cent.
Their most successful season was their first full campaign together, 2006-07. They started 33 of United's 60 games, winning 76 per cent. The following season, they started 41 of the 57 games, winning 68 per cent and ending it as champions of Europe.
But since 2009, the partnership has started to crumble.
Ferdinand has suffered injuries to his lower back, hamstring and calf. Vidic has undergone a serious knee operation.
Between 2006 and 2009—their first three full seasons together—they started together 106 times. In the next four—from 2009 to the end of last season—they started 57 games together.
Jonny Evans has been the main beneficiary, appearing 117 times for United since the start of the 2009-10 season. That's just three fewer than Ferdinand and four more than Vidic.
But it's only been this season that the partnership has started to look vulnerable. They were David Moyes' preferred pairing at the beginning of the season, starting the first six games together.
But since the 4-1 defeat to Manchester City at the Etihad Stadium, they've been restricted to just two games together, a Capital One Cup tie against Norwich and the Champions League game with Real Sociedad in Spain. Of the eight games they've played together, United have won just 50 per cent.
Vidic and Ferdinand still have a part to play at United. Moyes, in particular, has spoken of the importance of their vast experience.
But their days as "Ferdinand and Vidic" are coming to an end. Their role now is to ease in the next generation as "Ferdinand and Smalling" or "Vidic and Evans."
Time is the cruellest opponent for any footballer. It's the only one Ferdinand and Vidic can't tuck away in their pockets.