Two of the Premier League's greatest ever captains could leave their respective clubs this summer, but who will leave the biggest void?
The Serbian centre-back has been instrumental to the 15 trophies he has picked up in his time at Manchester United, which adds weight to argument of him being one of the real bargains of the Premier League era.
Officially signing for the club in January 2006 for a fee of just £7m, he has become one of the true greats of the modern Manchester United side.
He has won 5 Premier League titles, 3 League Cup titles and of course the Champions League win in 2008 and become known as one of the best Premier League players ever because of it.
However, with his contract up at the end of the year and no sign of an extension, just how big of a void will he leave for Moyes to fill?
There is no hiding from the fact his last few years have been ruined by injury and having missed large parts of the season for that reason, United have had to learn on some level to have a replacement at hand.
Yet with 15 Premier League games played this season, he has still proven himself to be a cut above the rest.
Defensively, there is nobody else in the team that makes as many effective clearances per game than him in game, no one who wins as many aerial battles than him and nobody who wins as many offsides than him per game.
He is tactically and defensively brilliant on all accounts and someone United should really push the boat out to keep on at all costs - a suitable replacement for such a talent and a leader will be very difficult to find, especially with Rio Ferdinand unlikely to be retained.
Former England captain John Terry has been the face of Chelsea during some of the best years in their history, but unfortunately that could end at the end of the season.
A Chelsea youth product that signed a professional deal at 17, could have never envisaged just quite the career he was going to have. He made his debut in 1998, coming on as a late substitute in a League Cup tie with Aston Villa.
14 trophies later, he can lay claim to being one of the most successful British captains in history, guiding Chelsea to domestic and European success. And if he is allowed to leave in the summer, he will no doubt receive a fanfare exit that his reputation deserves.
But just how big of a miss would he be for the modern day Chelsea?
This season he has been ever-present for Mourinho's side, playing in all 22 Premier League games so far. And like Vidic, he has made by far the most effective clearances in his respective team.
The goals that John Terry supplies will also be a big miss for Mourinho and replacing the clubs top goalscoring defender in history will be equally as hard as replacing his defensive contribution.
At 33, it was always going to be a key moment in his career and if he doesn't get a contract, I hope he doesn't throw the towel in. He is still arguably England's best central defender and can contribute at the top level for a few more years yet.
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